fix(a2a_client): surface error returns as tool errors through @tool decorator

[alisknox]

Description

Error returns from a2a_send_message (and other A2AClientToolProvider methods) included status='error' but no content key. The @tool decorator's _wrap_tool_result only preserves error status when both status and content keys are present. Without content, the error dict was wrapped as status='success' text content, preventing downstream hooks and middleware from detecting tool failures.

Add content key to all error returns while preserving existing keys for backward compatibility.

Related Issues

Fixes #606

Documentation PR

None

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
• My changes generate no new warnings
• Any dependent changes have been merged and published

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