fix: register_handoff is not working as expected in a_run_swarm

[karikalanarun]

Why are these changes needed?

I am doing one poc with a_run_swarm. I am just replicating the context router example using a_run_swarm. it didn't get transitioned properly. I got an error saying,

AttributeError: 'ConversableAgent' object has no attribute '_swarm_manager'

Closes #1603

Checks

• I've included any doc changes needed for https://docs.ag2.ai/. See https://docs.ag2.ai/docs/contributor-guide/documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

I haven't done anything in checks. But, I am thinking it's a minor fix, so.

[CLAassistant]

All committers have signed the CLA.

[kumaranvpl]

Hello @karikalanarun,

@sternakt shared fix for the CLA issue here - https://discord.com/channels/1153072414184452236/1359888693812400380/1360143512485625876

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

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HEAD has 1413 uploads less than BASE

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see 80 files with indirect coverage changes

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