Swarm: Restore lost SWARM_MANAGER return

[marklysze]

Why are these changes needed?

In PR #217, the return "auto" line was lost when merging with main in the last commit. This restores it.

Related issue number

N/A

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.

[marklysze]

Merging as the failed test is unrelated to this code.