Fix Rust connection callback context cleanup and add regression test

[LindonAliu]

Description

Closes: #5520

• Clean up connection callback contexts on ShutdownComplete so handlers set via ConnectionRef are freed automatically (no leak).
• Allow connection callbacks to return success when the context was already cleared (e.g., post-cleanup), avoiding panics.
• Add a regression test that asserts the server-side ConnectionRef handler is dropped after shutdown. Testing

Testing

• cargo test

Documentation

Is there any documentation impact for this change?

[LindonAliu]

@microsoft-github-policy-service agree

[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :warning: Please upload report for BASE (main@b69aa08). Learn more about missing BASE report. :warning: Report is 19 commits behind head on main.

Additional details and impacted files

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[LindonAliu]

Hi @guhetier! Thanks a lot for the review and approval. Since I don’t have merge permissions, could you please merge this PR when you get a chance? Thank you!

[guhetier]

I'll merge as soon as the CI completes (a few unrelated failures). Thanks @LindonAliu for the contribution!