perf: avoid copying the temp buffer in ByteStringBuilder.result

[pjfanning]

Motivation

ByteStringBuilder accumulates writes from putByte/putBytes/putInt/asOutputStream into a temporary array and flushes it in clearTemp, which always does java.util.Arrays.copyOf(_temp, _tempLength). On the final flush performed by result() that copy is avoidable: the buffer is not needed for further writes at that point, so every builder result pays a full copy of its tail chunk for nothing.

Modification

Add a private clearTempForResult used only by result():

• when more than half of the temp buffer holds data, the buffer is handed over to the resulting ByteString1 instead of being copied, and _temp/_tempCapacity are reset so that a later write to the builder allocates a fresh buffer rather than mutating the bytes that were handed over (every _temp write in the class goes through ensureTempSize first, and shouldResizeTempFor returns true on _tempCapacity == 0);
• when only a small part of the buffer is used it is still copied, so a small result does not retain a much larger array.

The mid-stream clearTemp calls made by addAll are unchanged, since the buffer is still reused there.

Result

No copy of the tail chunk on result() for builders that are at least half full.

A result produced by the hand-over path can be a non-compact ByteString1. That is copy-neutral: a consumer calling toArrayUnsafe on it pays exactly the copy that used to happen inside clearTemp, and it is avoided entirely for consumers that do not need an array. ByteStringUtils.toProtoByteStringUnsafe still takes its zero-copy asByteBuffers path for such a value.

Tests

• sbt "actor-tests/testOnly org.apache.pekko.util.ByteStringSpec" - 218 tests succeeded, 0 failed
• ByteStringSpec gains cases for the mostly-used, exactly-filled and small-write-in-a-large-buffer flush paths, that a returned ByteString is unaffected by later writes to the builder, and that the content is correct for every fill level from 0 to 200 bytes
• scalafmt --list --mode diff-ref=upstream/main - no files reported
• git diff --check - clean
• A broader run of actor-tests/testOnly org.apache.pekko.util.*, the stream io/compression/framing suites and actor/mimaReportBinaryIssues was started but stopped before completing, so it is not recorded as a pass. MiMa is expected to be unaffected: the change adds one private method and alters no signature.

References

None - follows the same no-copy approach as apache/pekko-http#1235