core: stop emitting legacy command events directly (#31629)

## Description

This PR removes the last path in core that emits `ExecCommandBegin` /
`ExecCommandEnd` directly.

Every command execution now starts and completes through canonical
`ItemStarted` / `ItemCompleted(TurnItem::CommandExecution)`. The
existing `HasLegacyEvent` compatibility layer still fans out Begin/End
afterward, so raw core event consumers and legacy rollout replay keep
seeing the same events.

`UnifiedExecInteraction` is dormant today. Live unified exec uses
`UnifiedExecStartup` for command lifecycle and `TerminalInteraction` for
`write_stdin` and polling, so this is code cleanup rather than a current
product behavior change. The main win is the code-level invariant where
all core flows emit `TurnItem` instead of legacy events.

## What changed

- Removed the `UnifiedExecInteraction` branches that emitted legacy
command events directly.
- Routed every command source through the existing canonical
`CommandExecution` lifecycle and compatibility fanout.

codex-rs/core/src/tools/events.rs

@@ -3,7 +3,6 @@ use crate::session::session::Session;
 use crate::session::turn_context::TurnContext;
 use crate::tools::context::SharedTurnDiffTracker;
 use crate::tools::sandboxing::ToolError;
-use crate::turn_timing::now_unix_timestamp_ms;
 use codex_apply_patch::AppliedPatchDelta;
 use codex_protocol::error::CodexErr;
 use codex_protocol::error::SandboxErr;
@@ -14,8 +13,6 @@ use codex_protocol::items::FileChangeItem;
 use codex_protocol::items::TurnItem;
 use codex_protocol::parse_command::ParsedCommand;
 use codex_protocol::protocol::EventMsg;
-use codex_protocol::protocol::ExecCommandBeginEvent;
-use codex_protocol::protocol::ExecCommandEndEvent;
 use codex_protocol::protocol::ExecCommandSource;
 use codex_protocol::protocol::ExecCommandStatus;
 use codex_protocol::protocol::FileChange;
@@ -104,25 +101,6 @@ pub(crate) async fn emit_exec_command_begin(
     interaction_input: Option<String>,
     process_id: Option<&str>,
 ) {
-    if matches!(source, ExecCommandSource::UnifiedExecInteraction) {
-        ctx.session
-            .send_event(
-                ctx.turn,
-                EventMsg::ExecCommandBegin(ExecCommandBeginEvent {
-                    call_id: ctx.call_id.to_string(),
-                    process_id: process_id.map(str::to_owned),
-                    turn_id: ctx.turn.sub_id.clone(),
-                    started_at_ms: now_unix_timestamp_ms(),
-                    command: command.to_vec(),
-                    cwd: cwd.clone(),
-                    parsed_cmd: parsed_cmd.to_vec(),
-                    source,
-                    interaction_input,
-                }),
-            )
-            .await;
-        return;
-    }
     ctx.session
         .emit_turn_item_started(
             ctx.turn,
@@ -568,32 +546,6 @@ async fn emit_exec_end(
     exec_input: ExecCommandInput<'_>,
     exec_result: ExecCommandResult,
 ) {
-    if matches!(exec_input.source, ExecCommandSource::UnifiedExecInteraction) {
-        ctx.session
-            .send_event(
-                ctx.turn,
-                EventMsg::ExecCommandEnd(ExecCommandEndEvent {
-                    call_id: ctx.call_id.to_string(),
-                    process_id: exec_input.process_id.map(str::to_owned),
-                    turn_id: ctx.turn.sub_id.clone(),
-                    completed_at_ms: now_unix_timestamp_ms(),
-                    command: exec_input.command.to_vec(),
-                    cwd: exec_input.cwd.clone(),
-                    parsed_cmd: exec_input.parsed_cmd.to_vec(),
-                    source: exec_input.source,
-                    interaction_input: exec_input.interaction_input.map(str::to_owned),
-                    stdout: exec_result.stdout,
-                    stderr: exec_result.stderr,
-                    aggregated_output: exec_result.aggregated_output,
-                    exit_code: exec_result.exit_code,
-                    duration: exec_result.duration,
-                    formatted_output: exec_result.formatted_output,
-                    status: exec_result.status,
-                }),
-            )
-            .await;
-        return;
-    }
     ctx.session
         .emit_turn_item_completed(
             ctx.turn,