fix: Substrait serialization mishandles sliced map literals

[neilconway]

Which issue does this PR close?

• N/A

Rationale for this change

ScalarValue::Map has an unfortunate property: it exposes the keys() and values() methods from the underlying Arrow MapArray, but those methods return all of the keys/values in the backing array, not just keys/values in the visible span. As a result, calling keys() on a ScalarValue::Map can return values outside the logical content of the scalar map.

Fix an instance of this bug in the substrait serialization code.

What changes are included in this PR?

• Fix substrait map literal serialization bug
• Add unit test

What is the testing strategy for this PR?

Existing tests pass, new test added.

Are there any user-facing changes?

No, aside from fixing the bug.

[neilconway]

Not sure if there's anything to be done about the confusing nature of the APIs here...

[codecov-commenter]

Codecov Report

❌ Patch coverage is 69.23077% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.39%. Comparing base (24a7f13) to head (0433f76).
⚠️ Report is 18 commits behind head on main.

Files with missing lines	Patch %	Lines
...ubstrait/src/logical_plan/producer/expr/literal.rs	69.23%	0 Missing and 8 partials ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##             main   #25543     +/-   ##
=========================================
  Coverage   82.38%   82.39%             
=========================================
  Files        1138     1138             
  Lines      433739   434744   +1005     
  Branches   433739   434744   +1005     
=========================================
+ Hits       357338   358193    +855     
- Misses      54847    54861     +14     
- Partials    21554    21690    +136     

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