Add regression test for stateless request memory cleanup

[felixweinberger]

Summary

This PR adds a regression test for the memory leak fix from PR #1116. The test ensures that transport resources are properly cleaned up after handling stateless requests in the StreamableHTTPSessionManager.

Problem

PR #1116 identified a memory leak where transport instances were not being terminated after handling stateless requests. This could lead to resource accumulation over time as streams and other resources were not properly closed.

Solution

The test verifies that:

• The transport's terminate() method is called after each stateless request
• The transport's internal state (_terminated flag and _request_streams) is properly cleaned up
• The test fails if the cleanup is not performed, providing regression protection

Test Implementation

The test creates real StreamableHTTPServerTransport instances and verifies the cleanup behavior rather than relying on mocks. This provides a more robust test that validates the actual behavior of the system.

Related to #1116

[felixweinberger]

@hopeful0 FYI

[hopeful0]

Thanks, I learned that the patch can helped us track down the transport, forgive my unfamiliarity with unittest.

[ihrpr]

Thank you