Allow setting daybreakEnabled when starting a thread (#45513)

## What changed

Add experimental `thread/start.daybreakEnabled` so clients can set the initial preference for persistent threads. Omitted or null values leave it unset; explicit values are rejected for ephemeral threads.

Return the choice in the start response, `thread/started`, and reads before persistence. Stage it with the initial thread metadata and save it when the thread is persisted. Later changes still use `thread/metadata/update`. The preference does not select `turn/start.cyberAccessProgram` or grant access.

## Testing

Add coverage for true, false, and unset values in responses, notifications, reads, and reads after persistence and restart, plus rejection for ephemeral threads. Update existing metadata and access-program tests to exercise threads with an initial preference.

GitOrigin-RevId: 3bff3dc55a18436067bc2a3f156f5abf52d7321b

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

@@ -126,6 +126,12 @@ pub struct ThreadStartParams {
     #[experimental("thread/start.projectId")]
     #[ts(optional = nullable)]
     pub project_id: Option<String>,
+    /// Initial Daybreak choice for this persistent thread. Omitted or null
+    /// leaves it unset. This does not select a turn's `cyberAccessProgram`
+    /// or grant access. Not supported for ephemeral threads.
+    #[experimental("thread/start.daybreakEnabled")]
+    #[ts(optional = nullable)]
+    pub daybreak_enabled: Option<bool>,
     /// Optional sticky environments for this thread.
     ///
     /// Omitted selects the default environment when environment access is