fix(channels): handle SDK messages stream events

[player0718]

Summary

• accept LangGraph SDK event: "messages" chunks in streaming channel responses
• preserve existing messages-tuple handling for compatibility
• update the Feishu channel streaming regression test to reproduce the SDK event shape

Why

Streaming IM channels request stream_mode=["messages-tuple", "values"], but the current LangGraph SDK emits message chunks with event: "messages". ChannelManager previously only accumulated messages-tuple, so Feishu/WeCom-style streaming users could miss incremental AI updates and only receive the later final value fallback.

Validation

• cd backend && PYTHONPATH=. uv run pytest tests/test_channels.py -q -k "stream or streaming"
• cd backend && PYTHONPATH=. uv run pytest tests/test_runtime_lifecycle_e2e.py tests/test_gateway_services.py -q
• cd backend && uv run ruff check app/channels/manager.py tests/test_channels.py
• cd backend && uv run ruff format --check app/channels/manager.py tests/test_channels.py
• git diff --check origin/main...HEAD

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