[codex-analytics] preserve user thread source for exec threads (#23376)

## Why
- Follows #20949.
- The above moved `thread_source` attribution from the reducer to
explicit caller provided metadata
- The `codex exec` path still omitted this metadata, leaving
exec-created threads without `thread_source`


## What Changed
- Ensures exec threads are marked as user created (`thread_source =
"user"`)
- Preserves thread-source metadata in exec’s startup session event


## Verification
- Updated unit tests to validate exec `thread_source` propagation.
- `cargo +1.93.0 test -p codex-exec --manifest-path codex-rs/Cargo.toml`
- `cargo +1.93.1 build -p codex-cli --manifest-path codex-rs/Cargo.toml`
- Validated locally with a freshly built `codex exec` run:
  - Startup logs showed `thread_source: Some(User)`.
  - Rollout metadata recorded `"thread_source":"user"`.

codex-rs/exec/src/lib_tests.rs

@@ -458,6 +458,25 @@ async fn thread_start_params_include_review_policy_when_auto_review_is_enabled()
     );
 }
 
+#[tokio::test]
+async fn thread_start_params_include_user_thread_source() {
+    let codex_home = tempdir().expect("create temp codex home");
+    let cwd = tempdir().expect("create temp cwd");
+    let config = ConfigBuilder::default()
+        .codex_home(codex_home.path().to_path_buf())
+        .fallback_cwd(Some(cwd.path().to_path_buf()))
+        .build()
+        .await
+        .expect("build config");
+
+    let params = thread_start_params_from_config(&config);
+
+    assert_eq!(
+        params.thread_source,
+        Some(codex_app_server_protocol::ThreadSource::User)
+    );
+}
+
 #[test]
 fn active_profile_selection_uses_profile_id_only() {
     let selection = permissions_selection_from_active_profile(ActivePermissionProfile::new(
@@ -548,6 +567,27 @@ async fn session_configured_from_thread_response_uses_permission_profile_from_co
     );
 }
 
+#[tokio::test]
+async fn session_configured_from_thread_response_preserves_thread_source() {
+    let codex_home = tempdir().expect("create temp codex home");
+    let cwd = tempdir().expect("create temp cwd");
+    let config = ConfigBuilder::default()
+        .codex_home(codex_home.path().to_path_buf())
+        .fallback_cwd(Some(cwd.path().to_path_buf()))
+        .build()
+        .await
+        .expect("build config");
+    let response = sample_thread_start_response();
+
+    let event = session_configured_from_thread_start_response(&response, &config)
+        .expect("build bootstrap session configured event");
+
+    assert_eq!(
+        event.thread_source,
+        Some(codex_protocol::protocol::ThreadSource::User)
+    );
+}
+
 fn sample_thread_start_response() -> ThreadStartResponse {
     ThreadStartResponse {
         thread: codex_app_server_protocol::Thread {
@@ -564,7 +604,7 @@ fn sample_thread_start_response() -> ThreadStartResponse {
             cwd: test_path_buf("/tmp").abs(),
             cli_version: "0.0.0".to_string(),
             source: codex_app_server_protocol::SessionSource::Cli,
-            thread_source: None,
+            thread_source: Some(codex_app_server_protocol::ThreadSource::User),
             agent_nickname: None,
             agent_role: None,
             git_info: None,