fix(wkt): panic in Duration deserialization

[coryan]

When deserializing a Duration string with more than 9 fractional digits,
&pad[s.len()..] would panic with an out-of-bounds byte index.

[gemini-code-assist]

Code Review

This pull request updates the Duration deserialization logic in src/wkt/src/duration.rs to handle fractional seconds exceeding 9 digits, adding validation for non-digit characters and empty nanoseconds, along with corresponding unit tests. The review feedback suggests optimizing the nanosecond parsing logic to avoid unnecessary heap allocations and string formatting overhead from format! by parsing the substring directly and mathematically scaling it.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.66%. Comparing base (86c2a65) to head (ed6ef8d).
⚠️ Report is 9 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6682   +/-   ##
=======================================
  Coverage   96.66%   96.66%           
=======================================
  Files         308      308           
  Lines       97778    97803   +25     
=======================================
+ Hits        94519    94544   +25     
  Misses       3259     3259           

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