Swarm: Support AfterWorkOption in SwarmResult agent parameter

[marklysze]

Why are these changes needed?

In line with Issue #357, adding further control of the next agent through the return of the SwarmResult in a tool call, would allow you to terminate, return to the user agent, use auto speaker selection next, or stay with the current agent after a tool call.

So you can now return an AfterWorkOption in the agent parameter of a SwarmResult, e.g.:

def publish_results(context_variables:dict) -> SwarmResult:
  ### take something from context_variables and turn it into a final string ###
  return SwarmResult(agent=AfterWorkOption.TERMINATE, context_variables=context_variables, values=final_string)

All AfterWorkOption's are supported.

Related issue number

Closes #357

Checks

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• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.