🧪 Add unit tests for action gate agents

[joy7758]

🎯 What: Added a missing test file tests/test_agents.py to cover the agent classes in src/titmas_action_gate/agents.py.
📊 Coverage: The test suite now includes comprehensive unit testing for:

• HandoffLog (adding entries and serialization)
• RequestAnalyst (analyzing action requests and ensuring validate_action_request is invoked)
• EvidenceVerifier (verifying interactions with ActionGateService)
• WorkflowLead (verifying evaluation requests to ActionGateService)
• GitHubOperator (verifying execute_allowed requests and proper provider injection)
• ReleaseSteward (verifying pull request merge parameters and agent ID overriding)
  ✨ Result: Test coverage for agents.py is established. This provides a safety net against regressions in agent structures or service invocation contracts.

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PR created automatically by Jules for task 15494024214090778476 started by @joy7758

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