utils: make timestamp validation strict

[tllovesxs]

Summary

• Make both timestamp validators reject invalid calendar dates.
• Require the complete input string to match the requested format.
• Add regression tests for leap-day boundaries, invalid dates, trailing input, and custom formats.

Fixes #33.

Test plan

• git diff --check
• Added TestTimeUtilsEdgeCases.
• Full Maven test execution is currently blocked by the repository's existing JAXB dependency issue on JDK 24.

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[zsphere-opensource]

Thanks for the detailed report and the comprehensive fix.
The changes correctly make both timestamp validators strict by rejecting invalid calendar dates and unconsumed trailing input. The added regression coverage for leap-day boundaries, invalid dates, trailing characters, and custom formats is helpful.
We’ll evaluate the implementation and run the relevant internal validation, including existing callers and null-input behavior. If the validation passes and the change is approved, we’ll integrate it through our internal workflow and synchronize the accepted commit to the public repository while preserving your authorship.
We’ll update this PR once the evaluation is complete.