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/app-server/src/in_process.rs

@@ -83,6 +83,7 @@ use codex_config::LoaderOverrides;
 use codex_config::ThreadConfigLoader;
 use codex_core::config::Config;
 use codex_core::init_state_db_from_config;
+use codex_core::resolve_installation_id;
 use codex_exec_server::EnvironmentManager;
 use codex_feedback::CodexFeedback;
 use codex_login::AuthManager;
@@ -345,7 +346,7 @@ impl InProcessClientHandle {
 /// the runtime is shut down and an `InvalidData` error is returned.
 pub async fn start(args: InProcessStartArgs) -> IoResult<InProcessClientHandle> {
     let initialize = args.initialize.clone();
-    let client = start_uninitialized(args).await;
+    let client = start_uninitialized(args).await?;
 
     let initialize_response = client
         .request(ClientRequest::Initialize {
@@ -365,12 +366,13 @@ pub async fn start(args: InProcessStartArgs) -> IoResult<InProcessClientHandle>
     Ok(client)
 }
 
-async fn start_uninitialized(args: InProcessStartArgs) -> InProcessClientHandle {
+async fn start_uninitialized(args: InProcessStartArgs) -> IoResult<InProcessClientHandle> {
     let channel_capacity = args.channel_capacity.max(1);
     let state_db = match args.state_db.clone() {
         Some(state_db) => Some(state_db),
         None => init_state_db_from_config(args.config.as_ref()).await,
     };
+    let installation_id = resolve_installation_id(&args.config.codex_home).await?;
     let (client_tx, mut client_rx) = mpsc::channel::<InProcessClientMessage>(channel_capacity);
     let (event_tx, event_rx) = mpsc::channel::<InProcessServerEvent>(channel_capacity);
 
@@ -438,6 +440,7 @@ async fn start_uninitialized(args: InProcessStartArgs) -> InProcessClientHandle
                 config_warnings: args.config_warnings,
                 session_source: args.session_source,
                 auth_manager,
+                installation_id,
                 rpc_transport: AppServerRpcTransport::InProcess,
                 remote_control_handle: None,
                 plugin_startup_tasks: crate::PluginStartupTasks::Start,
@@ -718,13 +721,13 @@ async fn start_uninitialized(args: InProcessStartArgs) -> InProcessClientHandle
         }
     });
 
-    InProcessClientHandle {
+    Ok(InProcessClientHandle {
         client: InProcessClientSender { client_tx },
         event_rx,
         runtime_handle,
         #[cfg(test)]
         _test_codex_home: None,
-    }
+    })
 }
 
 #[cfg(test)]