fix: support all packed value lengths in NumericFieldStats.decodeLong

[salvatorecampagna]

Problem

NumericFieldStats.decodeLong (introduced in #15760) only handled 4-byte (IntField) and 8-byte (LongField) packed point values via a switch on packed.length. Any other width caused an IllegalArgumentException, crashing the query for queries using byte lengths other than 4 or 8.

HalfFloatPoint produces 2-byte packed values. A range query on such a field triggered the exception. The original PR only tested IntField (4 bytes) and LongField (8 bytes), so CI did not catch the bug:

java.lang.IllegalArgumentException: Unsupported packed value length: 2 (expected 8 or 4)
    at org.apache.lucene.search.NumericFieldStats.decodeLong(NumericFieldStats.java:121)
    at org.apache.lucene.search.NumericFieldStats.getStatsFromPoints(NumericFieldStats.java:72)
    at org.apache.lucene.search.NumericFieldStats.getStats(NumericFieldStats.java:58)
    ...

Solution

Replace the switch-based decoder with a generic loop that handles any packed value length from 1 to 8 bytes. All Lucene point types use the same encoding: big-endian byte order with the sign bit flipped. The loop reads the bytes sequentially, re-flips the sign bit on the first byte, and sign-extends the result into a long. This is allocation-free, unlike NumericUtils.sortableBytesToBigInt, which copies the array and creates a BigInteger.

For point fields wider than 8 bytes (e.g. InetAddressPoint at 16 bytes, BigIntegerPoint at 16 bytes), getStatsFromPoints now returns null instead of throwing, allowing getStats to fall through to the DocValuesSkipper path. These wider point types are never used with SortedNumericDocValuesRangeQuery in practice, but the graceful fallback avoids unexpected failures.

Tests

• TestNumericFieldStats.testGetStatsWithAllByteWidths: exercises decodeLong with min, zero, and max values at every byte width from 1 to 8
• TestNumericFieldStats.testGetStatsReturnsNullForWidePointValues: verifies graceful null return for InetAddressPoint (16 bytes)
• TestHalfFloatPoint.testNumericFieldStats: integration test with real HalfFloatPoint (2 bytes)

./gradlew -p lucene/core test --tests "org.apache.lucene.search.TestNumericFieldStats"
./gradlew -p lucene/sandbox test --tests "org.apache.lucene.sandbox.document.TestHalfFloatPoint.testNumericFieldStats"

Follows up on #15760.

[romseygeek]

LGTM, thanks for updating @salvatore-campagna