Stream realtime V3 Codex handoff output (#33856)

## What changed

- Stream V3 agent message deltas to `delegation.context.append` at 200 ms intervals instead of waiting for each completed message.
- Send commentary without a prefix and label final or phase-less V1/V3 output with `"Agent Final Message":`.
- Bound streamed output while preserving both its beginning and final tail when truncation is required.
- Remove `codexResponseHandoffPrefix` from `thread/realtime/start`; unknown fields from older clients remain ignored.

## Testing

- Cover incremental V3 commentary and final output, Unicode-safe truncation, V1/V3 phase labeling, and legacy request compatibility.

GitOrigin-RevId: 633c18b5eb9a4f5756a264b63b7bfc185b073951

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

@@ -80,11 +80,6 @@ pub struct ThreadRealtimeStartParams {
     /// Optional prefix added to automatic Codex response items when `codexResponsesAsItems` is true.
     #[ts(optional = nullable)]
     pub codex_response_item_prefix: Option<String>,
-    /// Optional prefix added to automatic V1 or V3 Codex commentary sent through the selected
-    /// Bidi handoff wire event when `codexResponsesAsItems` is not true. Final answers are sent
-    /// without the prefix.
-    #[ts(optional = nullable)]
-    pub codex_response_handoff_prefix: Option<String>,
     /// Overrides the configured realtime model for this session only.
     #[ts(optional = nullable)]
     pub model: Option<String>,