AVRO-4267: [java] Compare bytes as unsigned

[1fanwang]

What is the purpose of the change

Java users who sort generic Avro records containing bytes can get a different order before and after serialization. Before this change, 0x7f sorted after 0xff in memory but before it in encoded data. After this change, both paths use Avro's unsigned byte order.

GenericData.compare now compares byte buffers over their remaining ranges with unsigned values and absolute reads. This preserves buffer positions and works with heap, sliced, read-only, and direct buffers. Converted logical types keep the existing Comparable behavior.

Fixes https://issues.apache.org/jira/browse/AVRO-4267

Verifying this change

This change adds coverage for unsigned values, prefixes, nonzero positions, sliced buffers, read-only buffers, direct buffers, position preservation, and converted logical types.

Raw logs

Before, on a9b88f538c74d2347444fac60d7289abcf231a50 with only the regression test applied:

$ JAVA_HOME=$(/usr/libexec/java_home -v 21) /opt/homebrew/bin/mvn -pl lang/java/avro -Dtest=TestCompare#genericBytesCompareMatchesBinaryUnsignedByteOrder test -DskipITs -DskipTests=false[ERROR] Tests run: 1, Failures: 1, Errors: 0, Skipped: 0[ERROR] TestCompare.genericBytesCompareMatchesBinaryUnsignedByteOrder:75 expected: <true> but was: <false>[INFO] BUILD FAILURE

After:

$ JAVA_HOME=$(/usr/libexec/java_home -v 21) /opt/homebrew/bin/mvn -Dmaven.build.cache.enabled=false -pl lang/java/avro clean test -DskipITs[INFO] Spotless.Java is keeping 315 files clean - 0 needs changes to be clean, 315 were already clean, 0 were skipped because caching determined they were already clean[INFO] Tests run: 3463, Failures: 0, Errors: 0, Skipped: 2[INFO] You have 0 Checkstyle violations.[INFO] Tests run: 3463, Failures: 0, Errors: 0, Skipped: 2[INFO] Tests run: 3463, Failures: 0, Errors: 0, Skipped: 2[INFO] BUILD SUCCESS

Documentation

• Does this pull request introduce a new feature? no
• If yes, how is the feature documented? not applicable

[prashantpiyush1111]

LGTM. Verified compareByteBuffers() — uses absolute get(index) reads so buffer positions are preserved (heap/sliced/direct/read-only, matches the tests), and the unsigned-compare logic correctly mirrors BinaryData.compareBytes(). Logical types (e.g. decimal) still fall through to the old Comparable path via the instanceof ByteBuffer guard. Good test coverage. No concerns, thanks!