Fix multiline paste in /goal edit (#26047)

Fixes #26025.

## Why
`/goal edit` opens `CustomPromptView`, which did not use the paste-burst
handling that protects the main composer when terminals deliver paste as
rapid key events. On Windows terminals, the first pasted newline could
be treated as Enter-to-submit, truncating the goal edit and leaving the
rest of the paste behind.

## What
This reuses `PasteBurst` in `CustomPromptView` as a lightweight
Enter-suppression detector for paste-like key streams. Characters still
insert directly, explicit paste still goes through the view paste path,
and ordinary text entry still submits on Enter.

codex-rs/tui/src/bottom_pane/paste_burst.rs

@@ -10,7 +10,7 @@
 //!   paste once enough chars have arrived.
 //!
 //! This module provides the `PasteBurst` state machine. `ChatComposer` feeds it only "plain"
-//! character events (no Ctrl/Alt) and uses its decisions to either:
+//! character events (no Ctrl/Alt) and uses the full buffering decisions to either:
 //!
 //! - briefly hold a first ASCII char (flicker suppression),
 //! - buffer a burst as a single pasted string, or
@@ -32,6 +32,10 @@
 //!   [`PasteBurst::flush_before_modified_input`] to avoid leaving buffered text "stuck", and then
 //!   [`PasteBurst::clear_window_after_non_char`] so subsequent typing does not get grouped into a
 //!   previous burst.
+//! - Direct-insert callers can skip buffering, use
+//!   [`PasteBurst::direct_insert_newline_should_insert`] in their Enter handler, and call
+//!   [`PasteBurst::extend_window`] when Enter or [`PasteBurst::on_plain_char_no_hold`] reports a
+//!   burst-like stream.
 //!
 //! # State Variables
 //!
@@ -106,10 +110,10 @@
 //! - [`PasteBurst::on_plain_char_no_hold`] never holds (used for IME/non-ASCII paths), since
 //!   holding a non-ASCII character can feel like dropped input.
 //!
-//! # Contract With `ChatComposer`
+//! # Contract With Callers
 //!
-//! `PasteBurst` does not mutate the UI text buffer on its own. The caller (`ChatComposer`) must
-//! interpret decisions and apply the corresponding UI edits:
+//! `PasteBurst` does not mutate the UI text buffer on its own. Callers must interpret decisions
+//! and apply the corresponding UI edits. `ChatComposer` uses the full buffering contract:
 //!
 //! - For each plain ASCII `KeyCode::Char`, call [`PasteBurst::on_plain_char`].
 //!   - [`CharDecision::RetainFirstChar`]: do **not** insert the char into the textarea yet.
@@ -333,6 +337,14 @@ impl PasteBurst {
         self.is_active() || in_burst_window
     }
 
+    /// Decide if Enter should insert a newline for callers that insert chars immediately.
+    pub fn direct_insert_newline_should_insert(&self, now: Instant) -> bool {
+        self.newline_should_insert_instead_of_submit(now)
+            || self
+                .last_plain_char_time
+                .is_some_and(|t| now.duration_since(t) <= PASTE_BURST_CHAR_INTERVAL)
+    }
+
     /// Keep the burst window alive.
     pub fn extend_window(&mut self, now: Instant) {
         self.burst_window_until = Some(now + PASTE_ENTER_SUPPRESS_WINDOW);