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/v2/realtime.rs

@@ -70,6 +70,12 @@ pub struct ThreadRealtimeStartParams {
     /// Overrides the configured realtime architecture for this session only.
     #[ts(optional = nullable)]
     pub architecture: Option<RealtimeConversationArchitecture>,
+    /// Sends automatic Codex responses as realtime conversation items instead of handoff appends.
+    #[ts(optional = nullable)]
+    pub codex_responses_as_items: Option<bool>,
+    /// Optional prefix added to automatic Codex response items when `codexResponsesAsItems` is true.
+    #[ts(optional = nullable)]
+    pub codex_response_item_prefix: Option<String>,
     /// Overrides the configured realtime model for this session only.
     #[ts(optional = nullable)]
     pub model: Option<String>,
@@ -146,6 +152,21 @@ pub struct ThreadRealtimeAppendTextParams {
 #[ts(export_to = "v2/")]
 pub struct ThreadRealtimeAppendTextResponse {}
 
+/// EXPERIMENTAL - append speakable text to thread realtime.
+#[derive(Serialize, Deserialize, Debug, Default, Clone, PartialEq, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(export_to = "v2/")]
+pub struct ThreadRealtimeAppendSpeechParams {
+    pub thread_id: String,
+    pub text: String,
+}
+
+/// EXPERIMENTAL - response for appending realtime speech.
+#[derive(Serialize, Deserialize, Debug, Default, Clone, PartialEq, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(export_to = "v2/")]
+pub struct ThreadRealtimeAppendSpeechResponse {}
+
 /// EXPERIMENTAL - stop thread realtime.
 #[derive(Serialize, Deserialize, Debug, Default, Clone, PartialEq, JsonSchema, TS)]
 #[serde(rename_all = "camelCase")]