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/message_processor.rs

@@ -259,6 +259,7 @@ pub(crate) struct MessageProcessorArgs {
     pub(crate) config_warnings: Vec<ConfigWarningNotification>,
     pub(crate) session_source: SessionSource,
     pub(crate) auth_manager: Arc<AuthManager>,
+    pub(crate) installation_id: String,
     pub(crate) rpc_transport: AppServerRpcTransport,
     pub(crate) remote_control_handle: Option<RemoteControlHandle>,
     pub(crate) plugin_startup_tasks: crate::PluginStartupTasks,
@@ -281,6 +282,7 @@ impl MessageProcessor {
             config_warnings,
             session_source,
             auth_manager,
+            installation_id,
             rpc_transport,
             remote_control_handle,
             plugin_startup_tasks,
@@ -302,6 +304,7 @@ impl MessageProcessor {
             state_db.clone(),
             Arc::clone(&thread_store),
             agent_graph_store.clone(),
+            installation_id,
         ));
         thread_manager
             .plugins_manager()