feat(mcp): per-tool options and progress update for MCPToolset

[longcw]

Summary

MCPToolset now extends AsyncToolset. Per-tool behavior is configured with MCPToolOptions, keyed by tool name via tool_options; any tool not listed there is a plain blocking call.

MCPToolOptions keeps the three concerns independent:

• flags: ToolFlag — e.g. CANCELLABLE, IGNORE_ON_ENTER (mirrors @function_tool).
• on_duplicate: DuplicateMode — duplicate-call policy (mirrors @function_tool).
• forward_progress: bool — forward the MCP server's report_progress notifications to ctx.update(), so the tool runs in the background and the agent can narrate progress.

When any tool is ToolFlag.CANCELLABLE, lk_agents_get_running_tasks / lk_agents_cancel_task are auto-exposed to the LLM by the activity.

AgentSession(tools=[
    MCPToolset(
        id="mcp",
        mcp_server=MCPServerHTTP(url="...", client_session_timeout_seconds=120),
        tool_options={
            "book_flight": MCPToolOptions(
                flags=ToolFlag.CANCELLABLE,
                on_duplicate="confirm",
                forward_progress=True,
            ),
        },
    ),
])

Example

examples/voice_agents/mcp/server.py ships a fast get_weather (sync, no progress) and a long-running book_flight that emits ctx.report_progress updates over ~70s. mcp-agent.py configures only book_flight (cancellable + forward progress) and bumps client_session_timeout_seconds=120 since the per-request timeout otherwise fires before it returns.

[theomonnom]

lgtm, let's have async_mode enabled by default?

maybe the arguments becomes something like nonblocking_tools: list[str]?

[theomonnom]

Let's wait for #5711