Add realtime speech append control (#27917)

## Why

Realtime voice harness tuning needs app-side control over what backend
Codex text is spoken. Backend orchestrator text is written for a reading
UI, so automatically speaking every preamble, progress update, or final
assistant message can make the realtime voice model too chatty.

For experimentation, clients need two simple controls: keep app/client
text-item injection on the existing item-create path, and add an
explicit speakable path that app code can call only when it wants
realtime to speak. Automatic Codex output also needs an opt-in way to
switch from the protocol's default speakable path to regular realtime
items, with a caller-provided prefix so prompt wording can be tuned
outside core.

The default remains unchanged: if a client omits the new start fields
and never calls `appendSpeech`, automatic backend output continues down
the existing speakable path for the selected realtime protocol.

## What Changed

- Adds experimental `thread/realtime/appendSpeech` for app-provided
speakable text.
- Keeps existing `thread/realtime/appendText` as the item-create API for
app-provided realtime text items.
- Adds `codexResponsesAsItems` / `codex_responses_as_items` on
`thread/realtime/start` to send automatic Codex responses with
`conversation.item.create` instead of the protocol's default speakable
output path.
- Adds `codexResponseItemPrefix` / `codex_response_item_prefix` so
clients can prepend experiment instructions to those automatic Codex
response items.
- Keeps literal `conversation.handoff.append` routing scoped to the v1
speakable path; v2 default speech uses its item/function-output plus
`response.create` behavior.
- Removes the earlier public silent-context API and hardcoded
silent-context prefix.
- Updates realtime tests to cover default automatic speakable behavior,
opt-in automatic item-create behavior, and explicit `appendSpeech`
behavior.

## Validation

- `cargo check -p codex-core -p codex-app-server -p codex-api`
- `just test -p codex-app-server realtime_conversation`
- `just test -p codex-core realtime_conversation` (50/51 passed in the
filtered parallel run; the lone failure passed when rerun in isolation)
- `just test -p codex-core
conversation_mirrors_assistant_message_text_to_realtime_handoff`
- `just test -p codex-api
e2e_connect_and_exchange_events_against_mock_ws_server`
- `just fix -p codex-core`
- `just fix -p codex-app-server`
- `cargo build -p codex-cli`

codex-rs/app-server/README.md

@@ -165,9 +165,10 @@ Example with notification opt-out:
 - `thread/inject_items` — append raw Responses API items to a loaded thread’s model-visible history without starting a user turn; returns `{}` on success.
 - `turn/steer` — add user input to an already in-flight regular turn without starting a new turn; returns the active `turnId` that accepted the input. `clientUserMessageId` is optional; when supplied, the corresponding `userMessage` item echoes it as `clientId`. Review and manual compaction turns reject `turn/steer`.
 - `turn/interrupt` — request cancellation of an in-flight turn by `(thread_id, turn_id)`; success is an empty `{}` response and the turn finishes with `status: "interrupted"`.
-- `thread/realtime/start` — start a thread-scoped realtime session (experimental); pass `outputModality: "text"` or `outputModality: "audio"` to choose model output, and optionally pass `model` and `version` to override configured realtime selection for this session only. Returns `{}` and streams `thread/realtime/*` notifications. Omit `transport` for the websocket transport, or pass `{ "type": "webrtc", "sdp": "..." }` to create a WebRTC session from a browser-generated SDP offer; the remote answer SDP is emitted as `thread/realtime/sdp`.
+- `thread/realtime/start` — start a thread-scoped realtime session (experimental); pass `outputModality: "text"` or `outputModality: "audio"` to choose model output, and optionally pass `model` and `version` to override configured realtime selection for this session only. By default, automatic Codex text follows the protocol's speakable output path. Pass `codexResponsesAsItems: true` to send automatic Codex responses as realtime conversation items instead, and optionally pass `codexResponseItemPrefix` to prepend experiment instructions to those items. Returns `{}` and streams `thread/realtime/*` notifications. Omit `transport` for the websocket transport, or pass `{ "type": "webrtc", "sdp": "..." }` to create a WebRTC session from a browser-generated SDP offer; the remote answer SDP is emitted as `thread/realtime/sdp`.
 - `thread/realtime/appendAudio` — append an input audio chunk to the active realtime session (experimental); returns `{}`.
 - `thread/realtime/appendText` — append text input to the active realtime session with a required `role` of `user` or `developer` (experimental); returns `{}`. Older clients that omit `role` default to `user`.
+- `thread/realtime/appendSpeech` — append text that the realtime model should speak to the user (experimental); returns `{}`.
 - `thread/realtime/stop` — stop the active realtime session for the thread (experimental); returns `{}`.
 - `review/start` — kick off Codex’s automated reviewer for a thread; responds like `turn/start` and emits `item/started`/`item/completed` notifications with `enteredReviewMode` and `exitedReviewMode` items, plus a final assistant `agentMessage` containing the review.
 - `command/exec` — run a single command under the server sandbox without starting a thread/turn (handy for utilities and validation).
@@ -878,6 +879,15 @@ Omit `prompt` to use Codex's default realtime backend prompt. Send `prompt: null
 `prompt: ""` when the session should start without that default backend prompt.
 Clients may also pass `model` and `version` on `thread/realtime/start` to select a
 different realtime session configuration without changing thread or user config.
+Pass `codexResponsesAsItems: true` to inject automatic Codex responses with
+`conversation.item.create` instead of the protocol's default speakable output
+path. When using that mode, `codexResponseItemPrefix` can prepend short
+experiment instructions to each automatic Codex response item. Omit
+`codexResponsesAsItems`, or pass `false`, to preserve the default speakable
+behavior. Call
+`thread/realtime/appendText` to append app-provided realtime text items, or
+`thread/realtime/appendSpeech` when the app decides a realtime update should be
+spoken.
 
 ```javascript
 await pc.setRemoteDescription({