feat: convert datetime values to RFC 3339 in WebDAV search literals

[bigcat88]

Fixes #347 .

This PR fixes incorrect handling of datetime and date objects in WebDAV find() queries. Previously, passing a datetime resulted in an invalid string format like 2025-03-10 12:34:56.123456, causing Nextcloud to misinterpret the value in <d:literal>.

Changes made:

• Added _dav_literal() helper to format datetime values as RFC 3339 strings (2025-03-10T12:34:56Z)
• Updated build_search_req() to use this helper when generating <d:literal> values

This ensures proper comparison logic for operators like "gt" and "lt" when filtering by last_modified or other time-based properties.

Fixes issues where files were incorrectly included or excluded due to string-based comparison.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.14%. Comparing base (ebce575) to head (a6828a7).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #358   +/-   ##
=======================================
  Coverage   95.13%   95.14%           
=======================================
  Files          45       45           
  Lines        5325     5333    +8     
=======================================
+ Hits         5066     5074    +8     
  Misses        259      259           

Files with missing lines	Coverage Δ
nc_py_api/files/_files.py	98.65% <100.00%> (+0.05%)	⬆️

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