fix: Don't panic in SQL temporal string check; raise suitable ColumnNotFound error

[alexander-beedie]

Closes #19426.

Fixes a rogue unwrap and adds some test coverage.

[codecov]

Codecov Report

Attention: Patch coverage is 90.90909% with 1 line in your changes missing coverage. Please review.

Project coverage is 79.94%. Comparing base (e7a84bc) to head (2958b57).
Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-sql/src/sql_expr.rs	90.90%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #19473   +/-   ##
=======================================
  Coverage   79.94%   79.94%           
=======================================
  Files        1534     1534           
  Lines      211183   211229   +46     
  Branches     2444     2445    +1     
=======================================
+ Hits       168831   168873   +42     
- Misses      41797    41801    +4     
  Partials      555      555           

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