fix(extensibility): update a2a-sdk Message serialization for protobuf-based types

[rishikunnath2747]

Description

a2a-sdk >= 1.0 changed a2a.types.Message from a Pydantic model to a protobuf-generated type. This broke two call sites in extensibility/client.py:

• message.model_dump(mode="json") — protobuf has no .model_dump(), causing a silent warning and empty message body
• Message(**response_json) — protobuf constructor rejects camelCase keys, silently dropping all fields from the hook response

Replaced with google.protobuf.json_format.MessageToDict and ParseDict respectively. Also bumped the a2a-sdk lower bound to >=1.0.0.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• Dependency update

How to Test

1. Install a2a-sdk>=1.0.0
2. Configure an agent with an n8n hook and trigger it
3. Verify the hook executes successfully and the response Message fields are correctly populated (previously all fields were silently dropped)

Checklist

• I have read the Contributing Guidelines
• I have verified that my changes solve the issue
• I have added/updated automated tests to cover my changes
• All tests pass locally
• I have verified that my code follows the Code Guidelines
• I have updated documentation (if applicable)
• I have added type hints for all public APIs
• My code does not contain sensitive information (credentials, tokens, etc.)
• I have followed Conventional Commits for commit messages