fix(streaming): skip control-only SSE blocks with no data

[lazerg]

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Changes being requested

An SSE block carrying only retry: or id: has no data, but the blank-line guard in SSEDecoder.decode() also looked at _last_event_id and _retry, so it dispatched an event with empty data anyway. Stream.__stream__() runs every event through sse.json(), and that empty event ends an otherwise valid stream with a JSON decoding error before the next data event is read. _last_event_id is kept across events on purpose, so once a stream has sent an id: field, any later blank line hits the same path.

The guard now dispatches only when an event name or data is buffered. Both fields stay on the decoder, so the next real event still reports the current id and retry.

Additional context & links

Closes #3833

tests/test_streaming.py::test_control_only_blocks_are_skipped fails on main and passes with this change. The streaming and SSE framing suites pass, and ruff check and format are clean on both touched files.

[lazerg]

Closing since the linked issue was closed. Happy to reopen if it turns out the fix is still wanted.