multiagent - use invoke_async instead of stream_async

[pgrayy]

Description

In our multiagent logic, just making a switch to agent.invoke_async from agent.stream_async in the spots we don't use the yielded events.

Related Issues

No issue, just a quality of life update.

Documentation PR

Internal only change.

Type of Change

Other (please describe): Code improvement.

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
• I ran hatch test tests_integ/test_multiagent_swarm.py and hatch test tests_integ/test_multiagent_graph.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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