HBASE-30391 Reduce array copying on expanding ByteArrayOutputStream

[Huginn-kio]

HBASE-30391

Summary

When ByteArrayOutputStream expands, it copies the entire backing array, including unused capacity. This change limits the copy to the bytes already written.

See https://issues.apache.org/jira/browse/HBASE-30391

Root cause

ByteArrayOutputStream.checkSizeAndGrow copied buf.length bytes when allocating a larger backing array, although only pos bytes contain valid stream data.

Fix

Copy only pos bytes during expansion. This preserves the stream contents while avoiding copies of unused capacity and reducing heap copy work.

Testing

• mvn -pl hbase-common -am -DskipTests compile with JDK 17: BUILD SUCCESS.
• Targeted Checkstyle for ByteArrayOutputStream.java: BUILD SUCCESS.
• The full reactor quality check was also attempted; it is currently blocked by pre-existing InterfaceIsType violations in untouched hbase-annotations test interfaces.

Compatibility

• No public API, configuration, persistence-format, or wire-format changes.