🛡️ Shield: Robust numeric timestamp parsing

[fderuiter]

🛑 Vulnerability: The parse_datetime validator implicitly assumed input timestamps would be strings (ISO format) or None. However, numeric timestamps (int/float seconds since epoch) are a valid data type in Python and potentially from API responses, but were being returned as-is (raw numbers) instead of converted to datetime objects. This bypasses type safety and could cause downstream AttributeErrors when methods like .strftime() are called on an integer.

🛡️ Defense: Updated parse_datetime in src/imednet/utils/validators.py to explicitly detect int and float types and convert them to timezone-aware UTC datetime objects using datetime.fromtimestamp(v, tz=timezone.utc).

🔬 Verification: Created tests/unit/test_parse_datetime_robustness.py which validates:

• Integer timestamps (e.g. 1609459200) -> UTC datetime
• Float timestamps (e.g. 1609459200.5) -> UTC datetime with microseconds
• Negative timestamps (historic dates) -> UTC datetime
• Zero/Epoch (legacy behavior) -> Returns sentinel date due to falsy check

📊 Impact: Fixes a potential "sad path" crash where numeric timestamps would be passed through as integers, ensuring 100% of outputs from parse_datetime are valid datetime objects (or the sentinel).

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PR created automatically by Jules for task 13020613294477168326 started by @fderuiter

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