Make the optional MCP startup grace configurable (#41199)

## What changed

- Add `mcp_optional_startup_grace_ms` with a default of 1,000 ms to control how long tool catalog capture waits for optional MCP servers.
- Treat a value of `0` as disabling the shared grace, so optional servers use their configured `startup_timeout_sec` instead.
- Apply updated grace values during runtime and MCP configuration refreshes, and reset cached startup deadlines when the configured duration changes.

## Testing

- Cover custom, disabled, refreshed, and shared startup grace behavior in MCP connection and core integration tests.

GitOrigin-RevId: 936196215ed6e0a29c9ec72cfee17663978df8ef

codex-rs/thread-manager-sample/src/main.rs

@@ -247,6 +247,7 @@ fn new_config(model: Option<String>, arg0_paths: Arg0DispatchPaths) -> anyhow::R
         mcp_oauth_credentials_store_mode: OAuthCredentialsStoreMode::File,
         mcp_oauth_callback_port: None,
         mcp_oauth_callback_url: None,
+        mcp_optional_startup_grace: std::time::Duration::from_secs(1),
         model_providers,
         project_doc_max_bytes: 32 * 1024,
         project_doc_fallback_filenames: Vec::new(),