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_tracing_tests.rs

@@ -298,6 +298,7 @@ async fn build_test_processor(
         config_warnings: Vec::new(),
         session_source: SessionSource::VSCode,
         auth_manager,
+        installation_id: "11111111-1111-4111-8111-111111111111".to_string(),
         rpc_transport: AppServerRpcTransport::Stdio,
         remote_control_handle: None,
         plugin_startup_tasks: crate::PluginStartupTasks::Start,