Stub MCP server used by Tasks lifecycle tests.

Declares the :tasks capability and exposes tools with different task_support values:

• wait_signal_add — :optional. Sends {:tool_running, self(), sig} to frame.assigns[:test_pid] and blocks on receive {:proceed, ^sig}. Used to coordinate tests deterministically without Process.sleep.
• must_be_task — :required. Echoes the message immediately.
• no_tasks — defaults to :forbidden. Returns "ok".
• always_fails — :optional. Returns a CallToolResult with isError: true.