[mcp] Support MCP Apps part 3 - Add mcp tool call support. (#17364)

- [x] Add a new app-server method so that MCP Apps can call their own
MCP server directly.

codex-rs/app-server/README.md

@@ -189,6 +189,7 @@ Example with notification opt-out:
 - `config/mcpServer/reload` — reload MCP server config from disk and queue a refresh for loaded threads (applied on each thread's next active turn); returns `{}`. Use this after editing `config.toml` without restarting the server.
 - `mcpServerStatus/list` — enumerate configured MCP servers with their tools and auth status, plus resources/resource templates for `full` detail; supports cursor+limit pagination. If `detail` is omitted, the server defaults to `full`.
 - `mcpServer/resource/read` — read a resource from a thread's configured MCP server by `threadId`, `server`, and `uri`, returning text/blob resource `contents`.
+- `mcpServer/tool/call` — call a tool on a thread's configured MCP server by `threadId`, `server`, `tool`, optional `arguments`, and optional `_meta`, returning the MCP tool result.
 - `windowsSandbox/setupStart` — start Windows sandbox setup for the selected mode (`elevated` or `unelevated`); accepts an optional absolute `cwd` to target setup for a specific workspace, returns `{ started: true }` immediately, and later emits `windowsSandbox/setupCompleted`.
 - `feedback/upload` — submit a feedback report (classification + optional reason/logs, conversation_id, and optional `extraLogFiles` attachments array); returns the tracking thread id.
 - `config/read` — fetch the effective config on disk after resolving config layering.