fix: session management on A2A server

[juliangrueber]

Description

• Added a method to the Strands Agent which creates a copy of itself with a new session manager and option to pass the request metadata defined by A2A into the agent state to use it downstream inside the tools

• Kept backwards compatibility

• Added a FileSessionManager as default to the server

• Added SessionManager factory method to create new sessions during request handling in A2A server

• To possible improvements (let me know if you prefer):

• more precise assertion for stream_async in in the unit test

• define a private method to attach the session manager to the hooks and reuse in the method

Related Issues

Closes #990

Documentation PR

Will provide once review is done

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

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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