Consolidate thread startup around StartThreadOptions (#34814)

## What changed

- Add `StartThreadOptions::new` to provide the standard configuration for a new thread.
- Make `ThreadManager::start_thread` the single thread-start entry point and migrate callers from the previous convenience methods.
- Derive default environment selections when `environments` is `None`, while preserving explicit selections, including an empty list.

GitOrigin-RevId: 8977dc11aed54c5e1215a81eaed2b2cf5fc6087a

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

@@ -2663,7 +2663,9 @@ mod tests {
             thread_id: conversation_id,
             thread: conversation,
             ..
-        } = thread_manager.start_thread(config.clone()).await?;
+        } = thread_manager
+            .start_thread(codex_core::StartThreadOptions::new(config.clone()))
+            .await?;
         let thread_state = new_thread_state();
         let thread_watch_manager = ThreadWatchManager::new();
         let (tx, mut rx) = mpsc::channel(CHANNEL_CAPACITY);
@@ -3241,7 +3243,9 @@ mod tests {
             thread_id: conversation_id,
             thread: conversation,
             ..
-        } = thread_manager.start_thread(config.clone()).await?;
+        } = thread_manager
+            .start_thread(codex_core::StartThreadOptions::new(config.clone()))
+            .await?;
         let thread_state = new_thread_state();
         {
             let mut state = thread_state.lock().await;
@@ -3329,7 +3333,9 @@ mod tests {
             thread_id: conversation_id,
             thread: conversation,
             ..
-        } = thread_manager.start_thread(config).await?;
+        } = thread_manager
+            .start_thread(codex_core::StartThreadOptions::new(config))
+            .await?;
         let child_thread_id = ThreadId::new();
         let child_thread_id_string = child_thread_id.to_string();
         let thread_watch_manager = ThreadWatchManager::new();
@@ -3419,7 +3425,9 @@ mod tests {
             thread_id: conversation_id,
             thread: conversation,
             ..
-        } = thread_manager.start_thread(config).await?;
+        } = thread_manager
+            .start_thread(codex_core::StartThreadOptions::new(config))
+            .await?;
         let (tx, mut rx) = mpsc::channel(CHANNEL_CAPACITY);
         let outgoing = Arc::new(OutgoingMessageSender::new(
             tx,

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

@@ -198,7 +198,10 @@ mod tests {
                 Some(temp_dir.path().join("good")),
             )
             .await?;
-        let thread = thread_manager.start_thread(thread_config).await?.thread;
+        let thread = thread_manager
+            .start_thread(codex_core::StartThreadOptions::new(thread_config))
+            .await?
+            .thread;
         std::fs::write(
             temp_dir.path().join(codex_config::CONFIG_TOML_FILE),
             r#"
@@ -337,8 +340,12 @@ enabled = false
                 /*external_time_provider*/ None,
             )
         });
-        thread_manager.start_thread(good_config).await?;
-        thread_manager.start_thread(bad_config).await?;
+        thread_manager
+            .start_thread(codex_core::StartThreadOptions::new(good_config))
+            .await?;
+        thread_manager
+            .start_thread(codex_core::StartThreadOptions::new(bad_config))
+            .await?;
 
         let loader = Arc::new(CountingThreadConfigLoader {
             good_cwd: AbsolutePathBuf::try_from(good_cwd)?,

codex-rs/app-server/src/request_processors/thread_processor.rs

@@ -1228,8 +1228,7 @@ impl ThreadRequestProcessor {
             ..
         } = listener_task_context
             .thread_manager
-            .start_thread_with_options(StartThreadOptions {
-                config,
+            .start_thread(StartThreadOptions {
                 allow_provider_model_fallback,
                 initial_history: match session_start_source
                     .unwrap_or(codex_app_server_protocol::ThreadStartSource::Startup)
@@ -1238,14 +1237,14 @@ impl ThreadRequestProcessor {
                     codex_app_server_protocol::ThreadStartSource::Clear => InitialHistory::Cleared,
                 },
                 history_mode,
-                session_source: None,
                 thread_source,
                 dynamic_tools,
                 metrics_service_name: service_name,
                 parent_trace: request_trace,
-                environments,
+                environments: Some(environments),
                 thread_extension_init,
                 supports_openai_form_elicitation,
+                ..StartThreadOptions::new(config)
             })
             .instrument(tracing::info_span!(
                 "app_server.thread_start.create_thread",