Add sticky environment API and thread state (#18897)

## Summary
- add sticky environment selections to app-server v2 thread/start and
turn/start request flow
- carry thread-level selections through core session/thread state
- add app-server coverage for sticky selections and turn overrides

## Stack
1. This PR: API and thread persistence
2. #18898: config.toml named environment loading
3. #18899: downstream tool/runtime consumers

## Validation
- Not run locally; split only.

---------

Co-authored-by: Codex <noreply@openai.com>

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

@@ -19,6 +19,7 @@ use codex_app_server_protocol::ThreadStartResponse;
 use codex_app_server_protocol::ThreadStartedNotification;
 use codex_app_server_protocol::ThreadStatus;
 use codex_app_server_protocol::ThreadStatusChangedNotification;
+use codex_app_server_protocol::TurnEnvironmentParams;
 use codex_config::types::AuthCredentialsStoreMode;
 use codex_core::config::set_project_trust_level;
 use codex_core::config_loader::project_trust_key;
@@ -48,6 +49,7 @@ use super::analytics::thread_initialized_event;
 use super::analytics::wait_for_analytics_payload;
 
 const DEFAULT_READ_TIMEOUT: std::time::Duration = std::time::Duration::from_secs(10);
+const INVALID_REQUEST_ERROR_CODE: i64 = -32600;
 
 #[tokio::test]
 async fn thread_start_creates_thread_and_emits_started() -> Result<()> {
@@ -166,6 +168,39 @@ async fn thread_start_creates_thread_and_emits_started() -> Result<()> {
     Ok(())
 }
 
+#[tokio::test]
+async fn thread_start_rejects_unknown_environment_as_invalid_request() -> Result<()> {
+    let server = create_mock_responses_server_repeating_assistant("Done").await;
+
+    let codex_home = TempDir::new()?;
+    create_config_toml_without_approval_policy(codex_home.path(), &server.uri())?;
+
+    let mut mcp = McpProcess::new(codex_home.path()).await?;
+    timeout(DEFAULT_READ_TIMEOUT, mcp.initialize()).await??;
+
+    let request_id = mcp
+        .send_thread_start_request(ThreadStartParams {
+            environments: Some(vec![TurnEnvironmentParams {
+                environment_id: "missing".to_string(),
+                cwd: codex_home.path().to_path_buf().try_into()?,
+            }]),
+            ..Default::default()
+        })
+        .await?;
+
+    let error: JSONRPCError = timeout(
+        DEFAULT_READ_TIMEOUT,
+        mcp.read_stream_until_error_message(RequestId::Integer(request_id)),
+    )
+    .await??;
+
+    assert_eq!(error.id, RequestId::Integer(request_id));
+    assert_eq!(error.error.code, INVALID_REQUEST_ERROR_CODE);
+    assert_eq!(error.error.message, "unknown turn environment id `missing`");
+
+    Ok(())
+}
+
 #[tokio::test]
 async fn thread_start_response_includes_loaded_instruction_sources() -> Result<()> {
     let server = create_mock_responses_server_repeating_assistant("Done").await;