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-protocol/src/protocol/common.rs

@@ -831,6 +831,12 @@ client_request_definitions! {
         serialization: thread_id(params.thread_id),
         response: v2::ThreadRealtimeAppendTextResponse,
     },
+    #[experimental("thread/realtime/appendSpeech")]
+    ThreadRealtimeAppendSpeech => "thread/realtime/appendSpeech" {
+        params: v2::ThreadRealtimeAppendSpeechParams,
+        serialization: thread_id(params.thread_id),
+        response: v2::ThreadRealtimeAppendSpeechResponse,
+    },
     #[experimental("thread/realtime/stop")]
     ThreadRealtimeStop => "thread/realtime/stop" {
         params: v2::ThreadRealtimeStopParams,
@@ -3032,6 +3038,8 @@ mod tests {
             request_id: RequestId::Integer(9),
             params: v2::ThreadRealtimeStartParams {
                 architecture: Some(RealtimeConversationArchitecture::Avas),
+                codex_responses_as_items: None,
+                codex_response_item_prefix: None,
                 thread_id: "thr_123".to_string(),
                 model: Some("realtime-treatment-model".to_string()),
                 output_modality: RealtimeOutputModality::Audio,
@@ -3049,6 +3057,8 @@ mod tests {
                 "params": {
                     "architecture": "avas",
                     "threadId": "thr_123",
+                    "codexResponsesAsItems": null,
+                    "codexResponseItemPrefix": null,
                     "model": "realtime-treatment-model",
                     "outputModality": "audio",
                     "prompt": "You are on a call",
@@ -3069,6 +3079,8 @@ mod tests {
             request_id: RequestId::Integer(9),
             params: v2::ThreadRealtimeStartParams {
                 architecture: None,
+                codex_responses_as_items: None,
+                codex_response_item_prefix: None,
                 thread_id: "thr_123".to_string(),
                 model: None,
                 output_modality: RealtimeOutputModality::Audio,
@@ -3086,6 +3098,8 @@ mod tests {
                 "params": {
                     "architecture": null,
                     "threadId": "thr_123",
+                    "codexResponsesAsItems": null,
+                    "codexResponseItemPrefix": null,
                     "model": null,
                     "outputModality": "audio",
                     "realtimeSessionId": null,
@@ -3101,6 +3115,8 @@ mod tests {
             request_id: RequestId::Integer(9),
             params: v2::ThreadRealtimeStartParams {
                 architecture: None,
+                codex_responses_as_items: None,
+                codex_response_item_prefix: None,
                 thread_id: "thr_123".to_string(),
                 model: None,
                 output_modality: RealtimeOutputModality::Audio,
@@ -3118,6 +3134,8 @@ mod tests {
                 "params": {
                     "architecture": null,
                     "threadId": "thr_123",
+                    "codexResponsesAsItems": null,
+                    "codexResponseItemPrefix": null,
                     "model": null,
                     "outputModality": "audio",
                     "prompt": null,
@@ -3166,6 +3184,29 @@ mod tests {
         Ok(())
     }
 
+    #[test]
+    fn serialize_thread_realtime_append_speech() -> Result<()> {
+        let request = ClientRequest::ThreadRealtimeAppendSpeech {
+            request_id: RequestId::Integer(10),
+            params: v2::ThreadRealtimeAppendSpeechParams {
+                thread_id: "thr_123".to_string(),
+                text: "Short voice update".to_string(),
+            },
+        };
+        assert_eq!(
+            json!({
+                "method": "thread/realtime/appendSpeech",
+                "id": 10,
+                "params": {
+                    "threadId": "thr_123",
+                    "text": "Short voice update"
+                }
+            }),
+            serde_json::to_value(&request)?,
+        );
+        Ok(())
+    }
+
     #[test]
     fn serialize_thread_status_changed_notification() -> Result<()> {
         let notification =
@@ -3276,6 +3317,8 @@ mod tests {
             request_id: RequestId::Integer(1),
             params: v2::ThreadRealtimeStartParams {
                 architecture: None,
+                codex_responses_as_items: None,
+                codex_response_item_prefix: None,
                 thread_id: "thr_123".to_string(),
                 model: None,
                 output_modality: RealtimeOutputModality::Audio,