Fix Parquet/Avro list struct MV expanded as array of maps

[rsrkpatwari1234]

Problem

When Parquet/Avro store a list as an array of structs with a single field "element", the record extractor was leaving them as arrays of single-key maps instead of unwrapping to plain arrays. For example:
Expected: "metadata": {"tags": ["abc", "xyz"]}
Actual: "metadata": {"tags": [{"element":"abc"}, {"element":"xyz"}]}
This breaks schemas and queries that expect multi-value columns to be arrays of scalars.

Solution

BaseRecordExtractor: After building a multi-value array from a Collection or Object[], run it through a new helper unwrapElementMapsInArray(). If every element is a Map with exactly one key "element", replace the array with the values of that key; otherwise leave the array unchanged. Do not apply unwrapping to primitive arrays.

Tests

Added BaseRecordExtractorTest with 8 cases covering unwrap when all elements are single-key "element" maps, no unwrap for mixed/multi-key/different-key/primitive/empty, and a full-path extract() test.

Scope

Change is in pinot-spi; any record reader that extends BaseRecordExtractor gets the fix.
Primitive arrays and non–element-map arrays are unchanged.

Fixes #17420

[codecov-commenter]

Codecov Report

❌ Patch coverage is 94.11765% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 63.20%. Comparing base (b0cb6cc) to head (42bd825).

Files with missing lines	Patch %	Lines
...he/pinot/spi/data/readers/BaseRecordExtractor.java	94.11%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #17708      +/-   ##
============================================
- Coverage     63.20%   63.20%   -0.01%     
  Complexity     1499     1499              
============================================
  Files          3179     3179              
  Lines        190696   190709      +13     
  Branches      29151    29155       +4     
============================================
+ Hits         120525   120532       +7     
- Misses        60797    60802       +5     
- Partials       9374     9375       +1     

Flag	Coverage Δ
custom-integration1	100.00% <ø> (ø)
integration	100.00% <ø> (ø)
integration1	100.00% <ø> (ø)
integration2	0.00% <ø> (ø)
java-11	63.16% <94.11%> (-0.02%)	⬇️
java-21	63.17% <94.11%> (+<0.01%)	⬆️
temurin	63.20% <94.11%> (-0.01%)	⬇️
unittests	63.19% <94.11%> (-0.01%)	⬇️
unittests1	55.63% <94.11%> (+0.01%)	⬆️
unittests2	34.04% <94.11%> (+<0.01%)	⬆️

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