Move installation ID resolution out of core startup (#21182)

## Summary

- resolve or inject the installation ID before core startup and pass it
through `ThreadManager`, `CodexSpawnArgs`, and `Session` as a plain
`String`
- keep child sessions on the parent installation ID instead of
rediscovering it inside core
- propagate installation ID startup failures in `mcp-server` instead of
panicking

## Why

Core was still touching the filesystem on the session startup path to
discover `installation_id`. This moves that work to the outer host
boundary so core no longer depends on `codex_home` reads during session
construction.

---------

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

codex-rs/core/src/thread_manager_tests.rs

@@ -1,5 +1,6 @@
 use super::*;
 use crate::config::test_config;
+use crate::installation_id::INSTALLATION_ID_FILENAME;
 use crate::rollout::RolloutRecorder;
 use crate::session::session::SessionSettingsUpdate;
 use crate::session::tests::make_session_and_context;
@@ -26,6 +27,8 @@ use std::time::Duration;
 use tempfile::tempdir;
 use wiremock::MockServer;
 
+const TEST_INSTALLATION_ID: &str = "11111111-1111-4111-8111-111111111111";
+
 fn user_msg(text: &str) -> ResponseItem {
     ResponseItem::Message {
         id: None,
@@ -415,6 +418,7 @@ async fn resume_and_fork_do_not_restore_thread_environments_from_rollout() {
         state_db,
         thread_store,
         agent_graph_store,
+        TEST_INSTALLATION_ID.to_string(),
     );
     let selected_cwd =
         AbsolutePathBuf::try_from(config.cwd.as_path().join("selected")).expect("absolute path");
@@ -509,6 +513,46 @@ async fn resume_and_fork_do_not_restore_thread_environments_from_rollout() {
     );
 }
 
+#[tokio::test]
+async fn explicit_installation_id_skips_codex_home_file() {
+    let temp_dir = tempdir().expect("tempdir");
+    let mut config = test_config().await;
+    config.codex_home = temp_dir.path().join("codex-home").abs();
+    config.cwd = config.codex_home.abs();
+    std::fs::create_dir_all(&config.codex_home).expect("create codex home");
+
+    let auth_manager =
+        AuthManager::from_auth_for_testing(CodexAuth::create_dummy_chatgpt_auth_for_testing());
+    let installation_id = uuid::Uuid::new_v4().to_string();
+    let (state_db, thread_store, agent_graph_store) = state_backed_stores(&config).await;
+    let manager = ThreadManager::new(
+        &config,
+        auth_manager,
+        SessionSource::Exec,
+        Arc::new(codex_exec_server::EnvironmentManager::default_for_tests()),
+        /*analytics_events_client*/ None,
+        state_db,
+        thread_store,
+        agent_graph_store,
+        installation_id.clone(),
+    );
+
+    let thread = manager
+        .start_thread(config.clone())
+        .await
+        .expect("start thread with explicit installation id");
+
+    assert!(!config.codex_home.join(INSTALLATION_ID_FILENAME).exists());
+    assert_eq!(thread.thread.codex.session.installation_id, installation_id);
+
+    thread
+        .thread
+        .shutdown_and_wait()
+        .await
+        .expect("shutdown thread");
+    let _ = manager.remove_thread(&thread.thread_id).await;
+}
+
 #[tokio::test]
 async fn resume_active_thread_from_rollout_returns_running_thread() {
     let temp_dir = tempdir().expect("tempdir");
@@ -529,6 +573,7 @@ async fn resume_active_thread_from_rollout_returns_running_thread() {
         state_db,
         thread_store,
         agent_graph_store,
+        TEST_INSTALLATION_ID.to_string(),
     );
 
     let source = manager
@@ -585,6 +630,7 @@ async fn resume_stopped_thread_from_rollout_spawns_new_thread() {
         state_db,
         thread_store,
         agent_graph_store,
+        TEST_INSTALLATION_ID.to_string(),
     );
 
     let source = manager
@@ -646,6 +692,7 @@ async fn resume_stopped_thread_from_rollout_preserves_thread_source() {
         state_db,
         thread_store,
         agent_graph_store,
+        TEST_INSTALLATION_ID.to_string(),
     );
 
     let source = manager
@@ -731,6 +778,7 @@ async fn new_uses_active_provider_for_model_refresh() {
         state_db,
         thread_store,
         agent_graph_store,
+        TEST_INSTALLATION_ID.to_string(),
     );
 
     let _ = manager.list_models(RefreshStrategy::Online).await;
@@ -945,6 +993,7 @@ async fn interrupted_fork_snapshot_does_not_synthesize_turn_id_for_legacy_histor
         state_db,
         thread_store,
         agent_graph_store,
+        TEST_INSTALLATION_ID.to_string(),
     );
 
     let source = manager
@@ -1051,6 +1100,7 @@ async fn interrupted_fork_snapshot_preserves_explicit_turn_id() {
         state_db,
         thread_store,
         agent_graph_store,
+        TEST_INSTALLATION_ID.to_string(),
     );
 
     let source = manager
@@ -1146,6 +1196,7 @@ async fn interrupted_fork_snapshot_uses_persisted_mid_turn_history_without_live_
         state_db,
         thread_store,
         agent_graph_store,
+        TEST_INSTALLATION_ID.to_string(),
     );
 
     let source = manager
@@ -1287,6 +1338,7 @@ async fn resumed_thread_keeps_paused_goal_paused() -> anyhow::Result<()> {
         state_db,
         thread_store,
         agent_graph_store,
+        TEST_INSTALLATION_ID.to_string(),
     );
 
     let source = manager