Allow callers to set the thread source for sampling (#56364)

## What changed

Add optional `threadSource` to `thread/sample` requests and apply it to the ephemeral fork. Keep `thread_sample` as the default when omitted.

## Testing

Extend sampling tests to assert that turn metadata includes the supplied thread source and retains the default when no source is provided.

GitOrigin-RevId: 8ea941c932267e3bcb9e6b086764ac2795d6edef

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

@@ -1,5 +1,6 @@
 //! One model response on an ephemeral fork that inherits the source's configuration and context.
 
+use super::ThreadSource;
 use crate::JsonSchema;
 use crate::RequestId;
 use crate::TS;
@@ -21,6 +22,8 @@ pub struct ThreadSampleParams {
     #[ts(optional = nullable)]
     pub effort: Option<ReasoningEffort>,
     #[ts(optional = nullable)]
+    pub thread_source: Option<ThreadSource>,
+    #[ts(optional = nullable)]
     pub turn_trigger: Option<String>,
     /// Optional metadata to enrich Codex's ResponsesAPI turn metadata.
     #[ts(optional = nullable)]

codex-rs/app-server/src/thread_sample.rs

@@ -213,7 +213,12 @@ impl SampleTask {
             model.supports_reasoning_effort_updates && options.config.model_provider.is_openai()
         });
         options.session_source = Some(SessionSource::Custom("thread_sample".to_string()));
-        options.thread_source = Some(ThreadSource::Feature("thread_sample".to_string()));
+        options.thread_source = Some(
+            params
+                .thread_source
+                .map(Into::into)
+                .unwrap_or_else(|| ThreadSource::Feature("thread_sample".to_string())),
+        );
         options.parent_trace = trace.clone();
         if cancellation.is_cancelled() {
             return Err(internal_error("sample canceled"));

codex-rs/app-server/tests/suite/v2/thread_sample.rs

@@ -16,6 +16,7 @@ use codex_app_server_protocol::ThreadLoadedListResponse;
 use codex_app_server_protocol::ThreadSampleCancelResponse;
 use codex_app_server_protocol::ThreadSampleParams;
 use codex_app_server_protocol::ThreadSampleResponse;
+use codex_app_server_protocol::ThreadSource;
 use codex_app_server_protocol::ThreadStartParams;
 use codex_app_server_protocol::TurnCompletedNotification;
 use codex_app_server_protocol::TurnInterruptParams;
@@ -304,6 +305,7 @@ async fn sample_preserves_context_and_keeps_source_unchanged(
             expected_turn_id: completed.turn.id.clone(),
             prompt: "Continue identically".to_string(),
             effort: Some(ReasoningEffort::Medium),
+            thread_source: Some(ThreadSource::Feature("composer_predictions".to_string())),
             turn_trigger: Some("composer_predictions".to_string()),
             responsesapi_client_metadata: Some(HashMap::from([
                 (
@@ -390,6 +392,7 @@ async fn sample_preserves_context_and_keeps_source_unchanged(
             .header("x-codex-turn-metadata")
             .expect("turn metadata"),
     )?;
+    assert_eq!(metadata["thread_source"], "composer_predictions");
     assert_eq!(metadata["turn_trigger"], "composer_predictions");
     assert_eq!(metadata["generation_trigger"], "composer_prediction");
     let body_metadata: Value = serde_json::from_str(
@@ -513,6 +516,7 @@ async fn sample_preserves_effort_baseline(
             .header("x-codex-turn-metadata")
             .expect("turn metadata"),
     )?;
+    assert_eq!(metadata["thread_source"], "thread_sample");
     assert!(metadata.get("turn_trigger").is_none());
     Ok(())
 }