refactor(a2a): async by default tools

[jer96]

Description

This PR ensures all A2A tools are async by default. Shoutout to @pgrayy for enabling this in the SDK!

Related Issues

• [Refactor] Remove run_async from a2a_client tool sdk-python#471

Documentation PR

• docs(a2a): revise a2a tool section docs#180

Type of Change

• refactor

Testing

• hatch fmt --linter
• hatch fmt --formatter
• hatch test --all
• Manual testing from steps from user guide: https://strandsagents.com/latest/documentation/docs/user-guide/concepts/multi-agent/agent-to-agent/#strands-a2a-tool

Checklist

• I have read the CONTRIBUTING document

• I have added 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

• My changes generate no new warnings

• Any dependent changes have been merged and published

• By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.