🧪 Add tests for timestamp validation edge cases

[sneakocom]

🎯 What: The testing gap for timestamp validation is now addressed.
📊 Coverage: The tests cover the happy path, missing Z timezone string, invalid object types, and exceptions when passed malformed ISO strings.
✨ Result: Improved test coverage across error conditions and guarantees deterministic check behaviour.

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PR created automatically by Jules for task 18117525502879257639 started by @sneakocom

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