perf: precompile formats in to_time

[lyne7-sc]

Which issue does this PR close?

• Closes #.

Rationale for this change

to_time previously parsed the Chrono format strings for every non-null input value. For array inputs, this repeated the same format parsing for each row in a batch.

This PR precompiles the format strings once per udf invocation and reuses the compiled format items while parsing each value.

What changes are included in this PR?

• Precompile and reuse the to_time format strings.
• Add tests for multiple formats and NULL values.

Are these changes tested?

Yes. Existing tests pass, and new slt coverage was added for multiple formats and NULL values.

Are there any user-facing changes?

No. This is an internal performance optimization.

Benchmarks

group                      main                            optimize-to-time
-----                       ----------                             ----------------
to_time_10pct_nulls_100k    1.83      7.3±0.34ms        ? ?/sec    1.00      4.0±0.47ms        ? ?/sec
to_time_no_nulls_100k       1.82      7.9±0.33ms        ? ?/sec    1.00      4.4±0.42ms        ? ?/sec

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.84%. Comparing base (e8a65f2) to head (643ce49).
⚠️ Report is 42 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #23964      +/-   ##
==========================================
+ Coverage   80.66%   80.84%   +0.17%     
==========================================
  Files        1095     1096       +1     
  Lines      372294   373848    +1554     
  Branches   372294   373848    +1554     
==========================================
+ Hits       300324   302247    +1923     
+ Misses      54055    53567     -488     
- Partials    17915    18034     +119     

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[neilconway]

Looks great, overall! Nicely targeted and well-written improvement, nice perf win.

[lyne7-sc]

Thanks for your review @neilconway!