fix(http): skip non-object JSON in SSE streams

[ningyixi]

Summary

If a connection-scoped SSE event contains valid JSON that is not an object (for example, data: []), the HTTP client passes it to JSON-RPC routing, which assumes a dictionary. The resulting exception closes the stream, and receive() returns None instead of delivering the next valid response.

Skip non-object payloads in the shared SSE decoder, just as it already skips invalid JSON. This matches the existing object filtering in the WebSocket and NDJSON transports. JSON objects continue through the normal JSON-RPC handling unchanged.

Validation

• Before the fix, the new tests produced 9 failures and 1 passing null control. All three HTTP-client cases returned EOF instead of the following session/new result.
• Parser coverage includes arrays, a batch-shaped array, strings, numbers, booleans, and null. HTTP-client regressions exercise the existing httpx2.MockTransport server and verify delivery of a valid response immediately after the invalid event.
• uv run python -m pytest tests/http/test_sse.py tests/http/test_http_client.py -q: 28 passed.
• make check: passed (lockfile, repository-wide pre-commit, Ruff, ty, deptry).
• make test: 448 passed, 1 skipped (optional Gemini CLI test).

No public API, configuration, or schema changes. The parser docstring documents the skip behavior. No live model calls were made.

AI assistance disclosure: Codex prepared the patch, tests, and this description, and ran the checks above. No human code review is claimed.