fix(collections/indexes): handle iterator close errors in index helpers

[songgaoye]

Description

Previously, iterator Close() errors were silently ignored in index helper
functions, which could lead to resource leaks (file descriptors, memory).
This change properly captures and returns close errors when no other error
has occurred, following the standard Go pattern for deferred cleanup.

Fixed functions:

• indexes.ScanKeyValues
• indexes.ScanValues
• indexes.ReversePairIterator.PrimaryKeys

Closes: #XXXX

[codecov]

Codecov Report

❌ Patch coverage is 84.90566% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 65.07%. Comparing base (3ca76ce) to head (90e7a8f).

Files with missing lines	Patch %	Lines
collections/indexes/helpers.go	84.00%	4 Missing ⚠️
collections/iter.go	87.50%	3 Missing ⚠️
collections/indexes/reverse_pair.go	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #25942      +/-   ##
==========================================
+ Coverage   64.87%   65.07%   +0.19%     
==========================================
  Files         890      907      +17     
  Lines       58583    60116    +1533     
==========================================
+ Hits        38008    39118    +1110     
- Misses      20575    20998     +423     

Files with missing lines	Coverage Δ
collections/indexes/reverse_pair.go	91.42% <75.00%> (ø)
collections/iter.go	86.74% <87.50%> (ø)
collections/indexes/helpers.go	92.15% <84.00%> (ø)

... and 52 files with indirect coverage changes

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[github-actions]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed in 7 days-before-close if no further activity occurs.

[songgaoye]

@aljo242 can you help review it? Thank you!