test: improve SSE streaming test reliability in CI environment

[OhYee]

Add real HTTP server support for SSE streaming tests to address CancelledError issues with httpx.ASGITransport in CI environments. Update test fixtures to use uvicorn-based servers instead of ASGI transport for more reliable stream handling during integration tests.

The changes replace httpx.ASGITransport with actual HTTP servers using uvicorn for testing SSE streams, which resolves premature cancellation problems in CI environments where ASGI transport failed to properly handle streaming responses.

这些更改将 httpx.ASGITransport 替换为使用 uvicorn 的实际 HTTP 服务器来测试 SSE 流，
解决了 CI 环境中 ASGI 传输无法正确处理流式响应导致的提前取消问题。

Change-Id: Ie47a62015e7530b2fd7e027e47e440d2fb3ee453

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