fix: flush the last SSE event when the stream ends without a blank line

[anxkhn]

Description

• Follow the CONTRIBUTING Guide
• Conventional Commits title
• Tests and linter pass
• Docs updated if necessary

Fixes #1213

Summary

parse_sse_stream only yields on a blank line. If the peer closes after data: {...} with no trailing blank line, leftover payload is dropped.

WHATWG SSE requires dispatching a pending event when the connection closes. JsonRpcTransport and RestTransport both use this parser, so a completed Task or Message can vanish and the client waits as if the stream is still open.

After the aiter_lines loop, if payload_chunks is non-empty, the pending event is now yielded the same way a blank line would. Existing tests already send a trailing newline and stay green.

Testing

Added test_parse_sse_stream_flushes_last_event_without_blank_line in tests/client/transports/test_http_helpers.py.

uv run pytest tests/client/transports/test_http_helpers.py -q

4 passed.

[github-actions]

🧪 Code Coverage (vs main)

⬇️ Download Full Report

	Base	PR	Delta
src/a2a/client/transports/http_helpers.py	95.54%	95.69%	🟢 +0.15%
Total	93.12%	93.12%	⚪️ 0.00%

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[mykytanetipa]

@anxkhn The rationale is inverted. WHATWG SSE (9.2.6, https://html.spec.whatwg.org/multipage/server-sent-events.html#event-stream-interpretation) requires the opposite: "Once the end of the file is reached, any pending data must be discarded. (If the file ends in the middle of an event, before the final empty line, the incomplete event is not dispatched.)"

So the parser is already correct, the client should discard an unterminated trailing event.