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/tests/common/test_codex.rs

@@ -18,6 +18,7 @@ use codex_core::ThreadManager;
 use codex_core::agent_graph_store_from_state_db;
 use codex_core::config::Config;
 use codex_core::init_state_db_from_config;
+use codex_core::resolve_installation_id;
 use codex_core::shell::Shell;
 use codex_core::shell::get_shell_by_model_provided_path;
 use codex_core::thread_store_from_config;
@@ -431,6 +432,7 @@ impl TestCodexBuilder {
                 .expect("test codex requires state db");
             let thread_store = thread_store_from_config(&config, state_db.clone());
             let agent_graph_store = agent_graph_store_from_state_db(state_db.clone());
+            let installation_id = resolve_installation_id(&config.codex_home).await?;
             ThreadManager::new(
                 &config,
                 codex_core::test_support::auth_manager_from_auth(auth.clone()),
@@ -440,6 +442,7 @@ impl TestCodexBuilder {
                 state_db,
                 thread_store,
                 agent_graph_store,
+                installation_id,
             )
         } else {
             codex_core::test_support::thread_manager_with_models_provider_and_home(