feat: support ansi for elt

[comphead]

Part of #23929

Rationale for this change

Spark's elt errors on out-of-range indices when ANSI is on, and returns NULL when off. DataFusion previously always returned NULL (there was a TODO for this in the source).

What changes are included in this PR?

• elt.rs: read enable_ansi_mode from ScalarFunctionArgs::config_options; raise "The index N is out of bounds. The array has M elements." in ANSI mode, keep NULL behavior otherwise. NULL index
  always returns NULL.
• elt.slt: add missing ANSI-off cases (index 0, negative, large, NULL index, NULL value, vectorized) and a full ANSI-on section with statement error assertions.

Are these changes tested?

Yes. New Rust unit tests and sqllogictest cases cover both modes.

Are there any user-facing changes?

With enable_ansi_mode = true, elt on invalid indices now errors instead of returning NULL. Default behavior unchanged. No API changes.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 94.20290% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.67%. Comparing base (2028927) to head (61ea765).

Files with missing lines	Patch %	Lines
datafusion/spark/src/function/string/elt.rs	94.20%	0 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #23928      +/-   ##
==========================================
- Coverage   80.67%   80.67%   -0.01%     
==========================================
  Files        1095     1095              
  Lines      372490   372542      +52     
  Branches   372490   372542      +52     
==========================================
+ Hits       300517   300557      +40     
- Misses      54035    54042       +7     
- Partials    17938    17943       +5     

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[Jefffrey]

do we need these tests if theyre already covered by slts?