fix: remove subscription on client disconnect in worker runtime

[MohMaz]

Why are these changes needed?

Potential conflict on subscriptions when agents disconnect on worker runtime. See explanations in https://github.com/microsoft/autogen/issues/3628

Related issue number

Closes https://github.com/microsoft/autogen/issues/3628

Checks

• [x] I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://microsoft.github.io/autogen/docs/Contribute#documentation to build and test documentation locally.
• [x] I've added tests (if relevant) corresponding to the changes introduced in this PR.
• [ ] I've made sure all auto checks have passed.

[MohMaz]

@ekzhu re-created the PR.

I'm not sure about I've made sure all auto checks have passed., it seems the current staging branch doesn't pass poe check. I did not fix things to keep the PR clean ...

I can create another PR to fix things to pass the staging pipelines once you have merged https://github.com/microsoft/autogen/pull/3651

[MohMaz]

cc: @jackgerrits in case you want to take a look following the conversation with had today