swarm - switch to handoff node only after current node stops

[pgrayy]

Description

Set the handoff node to current only after the current node finishes. Currently, we make the switch in the middle of the current node execution. It is important to fix this for a few reasons:

1. We emit the AfterNodeCallEvent with the current node id and state.current_node set to the handoff node. This is going to cause customer confusion.
2. If the current node runs a tool that is interrupted in parallel (concurrently) to the hand off tool, the swarm state will be invalid. The swarm state needs a reference to the real current node so that users can properly respond to its interrupts and resume execution.

Related Issues

#204

Documentation PR

Implementation detail

Type of Change

Bug fix

Testing

How have you tested the change? Verify that the changes do not break functionality or introduce warnings in consuming repositories: agents-docs, agents-tools, agents-cli

• I ran hatch run prepare: Relying on existing unit tests
• I ran hatch test tests_integ/test_multiagent_swarm.py

Checklist

• I have read the CONTRIBUTING document
• I have added any necessary tests that prove my fix is effective or my feature works
• I have updated the documentation accordingly
• I have added an appropriate example to the documentation to outline the feature, or no new docs are needed
• My changes generate no new warnings
• Any dependent changes have been merged and published

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[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 5 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/strands/multiagent/swarm.py	66.66%	3 Missing and 2 partials ⚠️

📢 Thoughts on this report? Let us know!

[dbschmigelski]

Can you add a unit test so we don't accidentally break the ordering in the future

[pgrayy]

Can you add a unit test so we don't accidentally break the ordering in the future

Done