Description

Since #1345, on_message_send raises InvalidAgentResponseError ("Agent execution produced no events") when the AgentExecutor returns without producing any event. on_message_send_stream did not have the same guard: its subscription ended with nothing yielded, so SendStreamingMessage answered 200 text/event-stream with zero events. Spec §3.1.2 requires the stream to begin with a Task or contain exactly one Message.

This PR applies the same check at the end of on_message_send_stream: when the subscription yielded no event and no task was created, it raises InvalidAgentResponseError. The JSON-RPC and REST dispatchers already read the first event eagerly, so the client now gets the same error as the non-streaming call (JSON-RPC -32006, REST 500 INVALID_AGENT_RESPONSE) instead of an empty stream. Executors that produce events are unaffected.

Tests: test_on_message_send_stream_raises_when_agent_produces_no_events reuses the SilentExecutor from the #1345 test.

$ pytest tests/server/request_handlers/test_default_request_handler_v2.py -k no_events   # handler from main
1 failed, 1 passed
$ pytest tests/server/request_handlers/test_default_request_handler_v2.py -k no_events   # this branch
2 passed
$ pytest tests/server tests/integration
1460 passed; the same 4 failures and 8 errors as on unmodified main (sample smoke tests and database tests that need services I don't have locally)

I also checked it over HTTP with a silent executor behind create_jsonrpc_routes and create_rest_routes: before, both streaming endpoints returned an empty 200 stream; after, JSON-RPC returns the -32006 error and REST returns 500 with reason INVALID_AGENT_RESPONSE.

ruff check, ruff format and ty check pass on the changed files.

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Fixes #1459 🦕

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