perf: Ensure string literals actually generate ColumnPredicates

[itamarst]

Another followup bug fix to the already merged #25412.

This time, ensuring that string literals actually generate column predicates. Lacking this PR, regexs do work, but nothing is generated for literals due to a bad assumption about strictness, so you don't get predicate pushdown.

Per Gijs this code is apparently used in streaming Parquet reading?

[codecov]

Codecov Report

❌ Patch coverage is 94.11765% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 82.17%. Comparing base (9250243) to head (1299cbe).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
...ars-plan/src/plans/aexpr/predicates/column_expr.rs	94.11%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #25495   +/-   ##
=======================================
  Coverage   82.17%   82.17%           
=======================================
  Files        1727     1727           
  Lines      240245   240262   +17     
  Branches     3033     3033           
=======================================
+ Hits       197410   197426   +16     
- Misses      42054    42055    +1     
  Partials      781      781           

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[ritchie46]

Do we miss an optimization opportunity, or do we actually read in rows we shouldn't? If the latter, can you come up with an MWE?

[itamarst]

I initially categorized it as a fix because the preexisting code was buggy and didn't work as intended. But... from a user perspective it's more of a missed optimization. So for changelog purposes I guess it's perf. I'll change the title for this and the other PR.

[itamarst]

I merged forward and fixed conflicts, so this should be ready for review again. The coverage failure is due to running out of disk space(!) and I can't rerun it with my permissions.