[2 of 3] Support long pasted text in TUI goals (#27509)

## Stack

1. [1 of 3] Support long raw TUI goal objectives - #27508
2. **[2 of 3] Support long pasted text in TUI goals** - this PR
3. [3 of 3] Support images in TUI goals - #27510

## Why

Large text pasted into the TUI composer is represented as a paste
placeholder plus pending paste metadata. For `/goal`, preserving only
the visible placeholder is not enough: the agent would see a short
placeholder string instead of the actual pasted text, and the long-text
support from the first PR would never see the payload.

The TUI also needs to avoid writing stale sidecar files when a user
pastes a large block and then deletes its placeholder before submitting
the goal.

## What Changed

- Introduces a TUI `GoalDraft` for goal submissions so `/goal`, `/goal
edit`, and queued goal commands can carry objective text plus text
elements and pending paste payloads.
- Materializes active pasted-text placeholders to `pasted-text-N.txt`
files through the app-server filesystem path introduced in #27508.
- Rewrites active paste placeholders in the persisted objective to file
references, while leaving literal placeholder-looking text alone.
- Filters out deleted paste placeholders so otherwise-small goals do not
require `$CODEX_HOME` or remote filesystem writes.
- Preserves pending paste metadata when a `/goal` command is queued
before a thread exists.

## Verification

- Added goal materialization tests for active paste placeholders,
deleted paste placeholders, and whitespace-only paste payloads.
- Added/updated TUI slash-command tests for large pasted text, queued
`/goal` commands before thread start, and queued oversized goal
behavior.

## Manual Testing

- Used real terminal bracketed-paste sequences through a remote TUI
session. A 1,228-byte multiline paste became `pasted-text-1.txt`; its
first/last lines and byte count matched exactly, and the persisted
objective referenced the server-host path.
- Pasted a large block, deleted its placeholder, and submitted a small
replacement objective. No new directory or sidecar file was created.
- Added two same-length large pastes to one goal. The composer
disambiguated their visible placeholders, and materialization preserved
order and contents in `pasted-text-1.txt` and `pasted-text-2.txt`.
- Submitted a whitespace-only large paste and verified the goal was
rejected as empty without writing a file.
- Submitted a pasted-text replacement while another goal was active,
verified no file was written before confirmation, then canceled and
confirmed the original goal remained unchanged.
- Combined a large paste with enough raw text to exceed 4,000 characters
after placeholder rewriting. The paste sidecar and `goal-objective.md`
were created in the same remote attachment directory, and `/goal edit`
restored the rewritten objective with its sidecar reference.

codex-rs/tui/src/chatwidget/slash_dispatch.rs

@@ -13,6 +13,7 @@ use crate::bottom_pane::slash_commands::ServiceTierCommand;
 use crate::bottom_pane::slash_commands::SlashCommandItem;
 use crate::bottom_pane::slash_commands::find_slash_command;
 use crate::goal_display::GOAL_USAGE;
+use crate::goal_files::GoalDraft;
 
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]
 enum SlashCommandDispatchSource {
@@ -23,6 +24,7 @@ enum SlashCommandDispatchSource {
 struct PreparedSlashCommandArgs {
     args: String,
     text_elements: Vec<TextElement>,
+    pending_pastes: Vec<(String, String)>,
     local_images: Vec<LocalImageAttachment>,
     remote_image_urls: Vec<String>,
     mention_bindings: Vec<MentionBinding>,
@@ -553,6 +555,22 @@ impl ChatWidget {
             return;
         }
 
+        if cmd == SlashCommand::Goal {
+            self.dispatch_prepared_command_with_args(
+                cmd,
+                PreparedSlashCommandArgs {
+                    args,
+                    text_elements,
+                    pending_pastes: self.bottom_pane.composer_pending_pastes(),
+                    local_images: Vec::new(),
+                    remote_image_urls: Vec::new(),
+                    mention_bindings: Vec::new(),
+                    source: SlashCommandDispatchSource::Live,
+                },
+            );
+            return;
+        }
+
         let Some((prepared_args, prepared_elements)) =
             self.prepare_live_inline_args(args, text_elements)
         else {
@@ -563,6 +581,7 @@ impl ChatWidget {
             PreparedSlashCommandArgs {
                 args: prepared_args,
                 text_elements: prepared_elements,
+                pending_pastes: Vec::new(),
                 local_images: Vec::new(),
                 remote_image_urls: Vec::new(),
                 mention_bindings: Vec::new(),
@@ -587,6 +606,7 @@ impl ChatWidget {
     fn clear_live_goal_submission(&mut self) {
         self.bottom_pane
             .set_composer_text(String::new(), Vec::new(), Vec::new());
+        self.bottom_pane.set_composer_pending_pastes(Vec::new());
         self.bottom_pane.drain_pending_submission_state();
     }
 
@@ -623,6 +643,7 @@ impl ChatWidget {
         let PreparedSlashCommandArgs {
             args,
             text_elements,
+            pending_pastes,
             local_images,
             remote_image_urls,
             mention_bindings,
@@ -750,18 +771,34 @@ impl ChatWidget {
                     }
                     return;
                 }
-                let objective = args.trim();
+                let draft = GoalDraft {
+                    objective: args,
+                    text_elements,
+                    pending_pastes,
+                };
                 let Some(thread_id) = self.thread_id else {
                     if source == SlashCommandDispatchSource::Live {
+                        const GOAL_PREFIX: &str = "/goal ";
+                        let text_elements = draft
+                            .text_elements
+                            .into_iter()
+                            .map(|element| {
+                                element.map_range(|range| ByteRange {
+                                    start: range.start + GOAL_PREFIX.len(),
+                                    end: range.end + GOAL_PREFIX.len(),
+                                })
+                            })
+                            .collect();
                         self.queue_user_message_with_options(
                             UserMessage {
-                                text: format!("/goal {args}"),
+                                text: format!("{GOAL_PREFIX}{}", draft.objective),
                                 local_images: Vec::new(),
                                 remote_image_urls: Vec::new(),
-                                text_elements: Vec::new(),
+                                text_elements,
                                 mention_bindings: Vec::new(),
                             },
                             QueuedInputAction::ParseSlash,
+                            draft.pending_pastes,
                         );
                         self.clear_live_goal_submission();
                     } else {
@@ -772,12 +809,13 @@ impl ChatWidget {
                     }
                     return;
                 };
-                self.app_event_tx.send(AppEvent::SetThreadGoalObjective {
+                let history_objective = draft.objective.clone();
+                self.app_event_tx.send(AppEvent::SetThreadGoalDraft {
                     thread_id,
-                    objective: objective.to_string(),
+                    draft,
                     mode: ThreadGoalSetMode::ConfirmIfExists,
                 });
-                self.append_message_history_entry(format!("/goal {trimmed}"));
+                self.append_message_history_entry(format!("/goal {history_objective}"));
                 if source == SlashCommandDispatchSource::Live {
                     self.clear_live_goal_submission();
                 }
@@ -831,7 +869,15 @@ impl ChatWidget {
         }
     }
 
-    pub(super) fn submit_queued_slash_prompt(&mut self, user_message: UserMessage) -> QueueDrain {
+    pub(super) fn submit_queued_slash_prompt(
+        &mut self,
+        queued_message: QueuedUserMessage,
+    ) -> QueueDrain {
+        let QueuedUserMessage {
+            user_message,
+            pending_pastes,
+            ..
+        } = queued_message;
         let UserMessage {
             text,
             local_images,
@@ -921,6 +967,7 @@ impl ChatWidget {
             PreparedSlashCommandArgs {
                 args: trimmed_rest.to_string(),
                 text_elements: args_elements,
+                pending_pastes,
                 local_images,
                 remote_image_urls,
                 mention_bindings,