Add maxValueCount to DocValuesSkipper metadata

[iprithv]

Description

resolves #15794

• Add DocValuesSkipper#maxValueCount() so consumers can determine whether a doc values field is definitely single-valued without unwrapping iterators.
• Persist exact global maxValueCount metadata in Lucene90 doc values skipper metadata for new segments.
• Add validation and test coverage for empty, single-valued, and multi-valued skipper fields.

Compatibility

Older Lucene90 segments do not have this metadata, so maxValueCount() returns a safe upper bound: 0 for empty fields and Integer.MAX_VALUE otherwise. Exact values become available once segments are rewritten or merged with the new format.

[jainankitk]

I am wondering if this information needs to stored as part of DocValuesSkipper as this should be single value per field per segment right? Also, this PR - #15737 might be related.

[iprithv]

I am wondering if this information needs to stored as part of DocValuesSkipper as this should be single value per field per segment right? Also, this PR - #15737 might be related.

Thanks for taking a look.

Yes, maxValueCount is a single global value per field per segment. This PR stores it that way: it is part of the Lucene90 DocValuesSkipperEntry field metadata, alongside the existing global minValue, maxValue, docCount, and maxDocId. It is not stored per skip interval/block.

I put it on DocValuesSkipper because this is where Lucene already exposes doc-values-derived segment-level metadata for fields that have a skip index. Putting it in FieldInfo would make it look like schema/static field metadata, but it is actually a computed per-segment doc values statistic and may be unavailable when no skipper is present.