fix(server): release the ActiveTask after a direct Message response - #1297

Description

A direct Message is a complete response for interactions that do not need task tracking (spec §3.1.1: the agent "MAY return a direct Message response for simple interactions"; §3.1.2: a message-only stream provides "No task tracking or updates"). No follow-up can continue such a request.

DefaultRequestHandlerV2 still kept the request's ActiveTask alive after a Message response:

• the producer stays parked on self._request_queue.get();
• the consumer and both EventQueueSource._dispatch_loop tasks wait with it;
• the entry stays in the ActiveTaskRegistry.

Every message-only request therefore leaves four pending asyncio tasks behind until the process exits. In a long-running server, the pending task count grows linearly with traffic.

Fix

After the consumer forwards the Message to subscribers, it finishes the ActiveTask the same way it already does for a terminal task state: it sets _is_finished and shuts down the request queue (_handle_terminal_state). The producer then leaves its loop and closes the event queues, the consumer drains and exits, and _maybe_cleanup removes the registry entry.

The Message is enqueued to subscribers before the shutdown, so on_message_send and on_message_send_stream still receive it.

Out of scope: input-required tasks stay parked for a follow-up, as covered by test_cancel_of_input_required_task_cannot_be_undone. This PR does not change that.

Tests

Two new tests in tests/server/request_handlers/test_default_request_handler_v2.py drive the real handler with an executor that answers with a single Message:

• test_on_message_send_message_response_releases_active_task: 3 × on_message_send, then no pending asyncio tasks and an empty registry;
• test_on_message_send_stream_message_response_releases_active_task: a message-only stream yields exactly one Message, then the same assertions.

Both fail on main (leftover ActiveTask._run_producer, ActiveTask._run_consumer and EventQueueSource._dispatch_loop) and pass with the fix.

Standalone repro, 50 requests each, counting pending asyncio tasks afterwards:

                main (ddce6b8)   this PR
completed             0              0
message             200              0
input-required      200            200   (unchanged, by design)

Checks

• ./scripts/lint.sh: passed. The 3 ty warnings are pre-existing, in client_factory.py and client/transports/__init__.py.
• uv run pytest: 2173 passed, 181 skipped, 3 xfailed, 1 xpassed.
• uv run pytest --cov=src: total 93 %, active_task.py 95 %; the new lines are covered.

Fixes #1296