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/tools/handlers/multi_agents_tests.rs

@@ -3171,6 +3171,7 @@ async fn tool_handlers_cascade_close_and_resume_and_keep_explicitly_closed_subtr
         state_db.clone(),
         thread_store_from_config(&config, state_db.clone()),
         agent_graph_store_from_state_db(state_db.clone()),
+        "11111111-1111-4111-8111-111111111111".to_string(),
     );
 
     let parent = manager