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/core/src/session/mod.rs

@@ -1800,6 +1800,7 @@ impl Session {
             config.tool_suggest =
                 resolve_tool_suggest_config_from_layer_stack(&config.config_layer_stack);
             config.mcp_servers = next_config.mcp_servers.clone();
+            config.mcp_optional_startup_grace = next_config.mcp_optional_startup_grace;
             config.mcp_oauth_credentials_store_mode = next_config.mcp_oauth_credentials_store_mode;
             if let Err(err) = config.features.set_enabled(
                 Feature::SecretAuthStorage,
@@ -1847,6 +1848,7 @@ impl Session {
             .config_layer_stack
             .with_user_layer_from(&config.config_layer_stack);
         config.mcp_servers = next_config.mcp_servers;
+        config.mcp_optional_startup_grace = next_config.mcp_optional_startup_grace;
         config.mcp_oauth_credentials_store_mode = next_config.mcp_oauth_credentials_store_mode;
         if let Err(err) = config.features.set_enabled(
             Feature::SecretAuthStorage,