fix(a2a): base64 decode byte data before placing in ContentBlocks

[dbschmigelski]

Description

This PR fixes a bug in the A2A executor where file bytes from A2A messages were not being decoded from base64 before passing to Strands agents. The A2A protocol sends file data as base64-encoded strings, but the executor was previously passing these encoded strings directly to the agent instead of decoding them back to raw bytes. This meant agents were receiving base64 text instead of actual file content. The fix adds proper base64 decoding with error handling to ensure agents receive the correct binary file data.

Related Issues

#850

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

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

✅ All modified and coverable lines are covered by tests.

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