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]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (main@b69aa08). Learn more about missing BASE report.
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5618   +/-   ##
=======================================
  Coverage        ?   85.28%           
=======================================
  Files           ?       60           
  Lines           ?    18663           
  Branches        ?        0           
=======================================
  Hits            ?    15917           
  Misses          ?     2746           
  Partials        ?        0           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[guhetier]

I think we have the same problem as above here.
ListenerClose could potentially trigger notifications (I don't think it currently does, but that might happen in the future). The context needs to stay set until ListenerClose returns.

[LindonAliu]

Thanks for the follow-up! I updated the close paths to keep the callback context alive until the Close call returns, so callbacks triggered synchronously during Close still see a valid context:

• Added peek_callback_ctx (read without clearing).
• Connection/Listener/Stream now use peek_callback_ctx in close_inner(), call Close, then drop the context after Close returns.

This aligns with the previous behavior you highlighted (context still present during Close) and prevents None when Close fires final notifications without a prior Shutdown.