fix: datetime verification evaluation about UTC in issuance and expiration

[peppelinux]

This pull request improves the handling of date-only validity periods in the MsoIssuer class by introducing a new date parsing method and updating the signing logic to ensure correct interpretation of expiry dates. It also adds a new test to verify that validity intervals are correctly set when issuance and expiry are on the same day.

Date parsing and validity handling improvements:

• Added a new static method parse_date to MsoIssuer in issuer.py to parse date-only strings and, when appropriate, set the time to the end of the day for expiry dates.
• Updated the sign method in MsoIssuer to use the new parse_date method for both issuance and expiry dates, ensuring expiry is interpreted as the end of the day.

Testing enhancements:

• Added a new test test_mso_issuer_validity_same_day to verify that when issuance and expiry dates are the same, the validity interval includes the entire day and the validUntil time is set to 23:59:59.
• Added necessary imports for datetime and cbor2 in the test file.

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 94.00000% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.31%. Comparing base (db4eb31) to head (92fcb33).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
pymdoccbor/tools.py	88.46%	3 Missing ⚠️
pymdoccbor/tests/test_06_mso_issuer.py	90.00%	2 Missing ⚠️
pymdoccbor/mso/verifier.py	75.00%	1 Missing ⚠️
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Additional details and impacted files

@@            Coverage Diff             @@
##             main      #36      +/-   ##
==========================================
+ Coverage   89.79%   90.31%   +0.52%     
==========================================
  Files          26       27       +1     
  Lines        1039     1126      +87     
==========================================
+ Hits          933     1017      +84     
- Misses        106      109       +3     

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