Add FieldMinMax unified API for global numeric min/max retrieval

[SYEDMDSAAD]

This patch introduces FieldMinMax, a unified API to retrieve global
minimum and maximum numeric values for a field across an IndexReader.

Currently Lucene exposes two different mechanisms:

• PointValues.getMinPackedValue / getMaxPackedValue

  • returns null when no values exist

• DocValuesSkipper.globalMinValue / globalMaxValue

  • returns sentinel values when metadata is unavailable

This forces callers to understand internal storage details and manually filter invalid values.

FieldMinMax abstracts over both implementations and provides consistent behavior:

• Uses PointValues when available (accurate index statistics)
• Falls back to DocValuesSkipper when metadata exists
• Falls back to scanning NumericDocValues when skipper metadata is absent
• Returns null when no values exist

This prevents sentinel leakage and simplifies caller logic.

Tests cover:

• missing field
• point values
• doc values
• mixed segments
• empty segments