fix: More robust str → date conversion when reading from spreadsheet

[alexander-beedie]

Closes #21068.

Strip any trailing zero-time components (eg: 00:00:00 or 00:00:00.000000) before applying .str.to_date(), when Date dtype is indicated.

We were doing this in some places already, but this PR ensures that we do it consistently across all supported engines, and adds some test coverage to validate.

[codecov]

Codecov Report

Attention: Patch coverage is 86.95652% with 3 lines in your changes missing coverage. Please review.

Project coverage is 80.76%. Comparing base (9f8976f) to head (d02213f).
Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
py-polars/polars/io/spreadsheet/functions.py	86.95%	0 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #22276      +/-   ##
==========================================
- Coverage   80.77%   80.76%   -0.02%     
==========================================
  Files        1640     1640              
  Lines      235594   235827     +233     
  Branches     2714     2720       +6     
==========================================
+ Hits       190302   190462     +160     
- Misses      44651    44722      +71     
- Partials      641      643       +2     

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