[codex] Flush trailing realtime transcript tail (#29918)

## Why

A realtime session can end after more transcript has accumulated than
was included in its last handoff. That tail already lives in core's
active transcript state, but the stop path aborted the realtime
input/fanout tasks before routing it, so the final bit of the
conversation could disappear before `thread/realtime/closed`.

This behavior is still being evaluated, so clients must opt in per
realtime session. Omitted or false leaves shutdown behavior unchanged.

## What changed

- Add optional `flushTranscriptTailOnSessionEnd` to
`thread/realtime/start`, defaulting to false in app-server.
- Expose an idempotent `take_transcript_tail()` from the existing active
transcript state using `last_handoff_entry_count`.
- When enabled, let shutdown cancel the input owner cleanly and publish
at most one final existing `<realtime_delegation>` with the remaining
text in `<transcript_delta>`.
- Have the existing fanout drain already-parsed events before routing
that final delegation, so a queued handoff wins first and is not
duplicated in the tail.
- Flush realtime shutdown before ordinary session task abort during core
cleanup.

## Validation

- `just test -p codex-app-server-protocol`
- `just test -p codex-app-server
realtime_conversation_stop_emits_closed_notification`
- `just test -p codex-core conversation_transport_close_`
- `just test -p codex-core
conversation_close_routes_only_remaining_transcript_tail_once`
- scoped `just fix` for `codex-app-server-protocol`, `codex-app-server`,
`codex-protocol`, and `codex-core`
- `just fmt`

codex-rs/app-server-protocol/src/protocol/v2/realtime.rs

@@ -70,6 +70,10 @@ pub struct ThreadRealtimeStartParams {
     /// them automatically. Defaults to false.
     #[ts(optional = nullable)]
     pub client_managed_handoffs: Option<bool>,
+    /// Routes any transcript tail remaining at session end through Codex. Defaults to false.
+    /// TODO: Remove this rollout knob once transcript-tail flushing is always enabled.
+    #[ts(optional = nullable)]
+    pub flush_transcript_tail_on_session_end: Option<bool>,
     /// Sends automatic Codex responses as realtime conversation items instead of handoff appends.
     #[ts(optional = nullable)]
     pub codex_responses_as_items: Option<bool>,