test(langchain): add real HTTP server support for agent invoke tests

[OhYee]

Add _start_server function to launch actual FastAPI servers for integration testing, modify request functions to accept both app objects and server URLs, and update test fixtures to use real HTTP connections instead of ASGI transport.

This enables more realistic testing of agent invoke methods using actual HTTP requests while maintaining backward compatibility with ASGI transport for non-streaming tests.

The changes affect test_agent_invoke_methods.py with new server startup logic, enhanced request handling, and proper server cleanup in test fixtures.

test(langchain): 为代理调用测试添加真实 HTTP 服务器支持

添加 _start_server 函数以启动实际的 FastAPI 服务器进行集成测试，
修改请求函数以接受 app 对象和服务器 URL，
并更新测试 fixture 以使用真实的 HTTP 连接而不是 ASGI 传输。

这使得使用实际 HTTP 请求对代理调用方法进行更真实的测试
同时保持与 ASGI 传输的向后兼容性以用于非流式测试。

这些更改影响 test_agent_invoke_methods.py 中的新服务器启动逻辑、
增强的请求处理和测试 fixture 中的适当服务器清理。

Change-Id: Icb5b09c2d588021cf72749658b1d07237fc1ef9c

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