Add integration test for aconnect_sse using FastAPI and httpx (Fixes #109)

[Vinisha-projects]

This PR adds an integration test for aconnect_sse() in mcp.client.sse. It connects to a mock FastAPI SSE server and verifies that the client receives the expected streamed events.

Motivation and Context

This test addresses Issue #109, which requests automated test coverage for the mcp.client.sse module.
It ensures the aconnect_sse() function works correctly with real SSE server behavior.

How Has This Been Tested?

1. Verified locally in a Jupyter notebook and .py script
2. The test connects to a FastAPI SSE endpoint, collects 3 streamed events, and asserts correct message order and content
3. Screenshot attached below from successful test run

Breaking Changes

No breaking changes introduced. This is a non-intrusive test addition.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context