fix(multiagent): remove a2a module from strands.multiagent __init__.py as it is an optional dependency

[awsarron]

Description

remove a2a module from strands.multiagent init.py as it is an optional dependency

without this fix, importing anything from strands.multiagent module requires the a2a optional dependencies

Related Issues

N/A

Documentation PR

N/A

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
• Ran hatch run test-integ
• Installed strands-agents in a fresh venv, tested with from strands.multiagent import GraphBuilder in a standalone script. Previously it failed requiring the a2a dependencies, now it works fine

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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[awsarron]

duplicate of #447