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/mcp-server/src/message_processor.rs

@@ -55,6 +55,7 @@ impl MessageProcessor {
         arg0_paths: Arg0DispatchPaths,
         config: Arc<Config>,
         environment_manager: Arc<EnvironmentManager>,
+        installation_id: String,
     ) -> Option<Self> {
         let outgoing = Arc::new(outgoing);
         let auth_manager = AuthManager::shared_from_config(
@@ -74,6 +75,7 @@ impl MessageProcessor {
             state_db,
             thread_store,
             agent_graph_store,
+            installation_id,
         ));
         Some(Self {
             outgoing,