feat(analytics): generate an installation_id and pass it in responsesapi client_metadata (#16912)

## Summary

This adds a stable Codex installation ID and includes it on Responses
API requests via `x-codex-installation-id` passed in via the
`client_metadata` field for analytics/debugging.

The main pieces are:
- persist a UUID in `$CODEX_HOME/installation_id`
- thread the installation ID into `ModelClient`
- send it in `client_metadata` on Responses requests so it works
consistently across HTTP and WebSocket transports

codex-rs/core/src/codex_tests.rs

@@ -255,6 +255,7 @@ fn test_model_client_session() -> crate::client::ModelClientSession {
         /*auth_manager*/ None,
         ThreadId::try_from("00000000-0000-4000-8000-000000000001")
             .expect("test thread id should be valid"),
+        /*installation_id*/ "11111111-1111-4111-8111-111111111111".to_string(),
         ModelProviderInfo::create_openai_provider(/* base_url */ /*base_url*/ None),
         codex_protocol::protocol::SessionSource::Exec,
         /*model_verbosity*/ None,
@@ -2772,6 +2773,7 @@ pub(crate) async fn make_session_and_context() -> (Session, TurnContext) {
         model_client: ModelClient::new(
             Some(auth_manager.clone()),
             conversation_id,
+            /*installation_id*/ "11111111-1111-4111-8111-111111111111".to_string(),
             session_configuration.provider.clone(),
             session_configuration.session_source.clone(),
             config.model_verbosity,
@@ -3613,6 +3615,7 @@ pub(crate) async fn make_session_and_context_with_dynamic_tools_and_rx(
         model_client: ModelClient::new(
             Some(Arc::clone(&auth_manager)),
             conversation_id,
+            /*installation_id*/ "11111111-1111-4111-8111-111111111111".to_string(),
             session_configuration.provider.clone(),
             session_configuration.session_source.clone(),
             config.model_verbosity,