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/mcp-server/src/codex_tool_runner.rs

@@ -11,6 +11,7 @@ use crate::outgoing_message::OutgoingNotificationMeta;
 use crate::patch_approval::handle_patch_approval_request;
 use codex_core::CodexThread;
 use codex_core::NewThread;
+use codex_core::StartThreadOptions;
 use codex_core::ThreadManager;
 use codex_core::config::Config as CodexConfig;
 use codex_protocol::ThreadId;
@@ -66,7 +67,10 @@ pub async fn run_codex_tool_session(
         thread_id,
         thread,
         session_configured,
-    } = match thread_manager.start_thread(config.clone()).await {
+    } = match thread_manager
+        .start_thread(StartThreadOptions::new(config.clone()))
+        .await
+    {
         Ok(res) => res,
         Err(e) => {
             let result = CallToolResult::error(vec![Content::text(format!(