🧪 Add tests for runtime admission logic

[joy7758]

🎯 What: Created a missing test file for src/titmas_action_gate/runtime.py covering key functionality like RuntimePrincipal, RuntimePrincipalRegistry, and agent_registry_path.
📊 Coverage: Covered happy paths (default and configured registry paths, successful principal creation/loading) and edge cases/errors (registry file not found, bad credential file permissions, insufficient credential length).
✨ Result: Increased code test coverage and reliability for runtime admission logic by filling a known coverage gap.

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PR created automatically by Jules for task 2265017764240553073 started by @joy7758

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