fix(client): surface streamable HTTP request failures

[Genmin]

Fixes #915

Summary

• convert per-request Streamable HTTP transport failures into JSON-RPC errors for the pending request
• keep notification transport failures contained so the transport task group does not crash
• add regressions for direct streamable_http_client use and ClientSessionGroup.connect_to_server()

Validation

• uv run pytest tests/client/test_notification_response.py::test_transport_error_sends_jsonrpc_error tests/client/test_notification_response.py::test_transport_error_on_notification_does_not_crash_transport tests/client/test_session_group.py::test_client_session_group_streamable_http_connect_error_is_catchable -q
• uv run pytest tests/client/test_notification_response.py tests/client/test_session_group.py -q
• uv run pytest tests/shared/test_streamable_http.py::test_streamable_http_client_basic_connection tests/shared/test_streamable_http.py::test_streamable_http_client_error_handling tests/client/test_transport_stream_cleanup.py::test_streamable_http_client_closes_all_streams_on_exit -q
• uv run ruff format --check src/mcp/client/streamable_http.py tests/client/test_notification_response.py tests/client/test_session_group.py
• uv run ruff check src/mcp/client/streamable_http.py tests/client/test_notification_response.py tests/client/test_session_group.py
• uv run pyright src/mcp/client/streamable_http.py tests/client/test_notification_response.py tests/client/test_session_group.py
• git diff --check

[Genmin]

Pushed a small CI cleanup after inspecting the failed jobs. The original regression test covered previously excluded ClientSessionGroup cleanup/error paths, so I narrowed those coverage pragmas and reran the CI-equivalent coverage checks locally.\n\nValidation run after the update:\n- uv run pytest tests/client/test_notification_response.py tests/client/test_session_group.py -q\n- uv run ruff format --check src/mcp/client/streamable_http.py src/mcp/client/session_group.py tests/client/test_notification_response.py tests/client/test_session_group.py\n- uv run ruff check src/mcp/client/streamable_http.py src/mcp/client/session_group.py tests/client/test_notification_response.py tests/client/test_session_group.py\n- uv run pyright src/mcp/client/streamable_http.py src/mcp/client/session_group.py tests/client/test_notification_response.py tests/client/test_session_group.py\n- uv run --frozen --no-sync coverage run -m pytest -n auto && uv run --frozen --no-sync coverage combine || true; uv run --frozen --no-sync coverage report && uv run --frozen --no-sync strict-no-cover\n- UV_PROJECT_ENVIRONMENT=.venv-py314 uv run --python 3.14 pytest tests/client/test_notification_response.py::test_transport_error_sends_jsonrpc_error tests/client/test_session_group.py::test_client_session_group_streamable_http_connect_error_is_catchable -q