perf: optimise UnsynchronizedByteArrayInputStream bulk operations

[pjfanning]

Motivation

UnsynchronizedByteArrayInputStream (used by ByteString.asInputStream and the Java/Jackson serializers) only overrode the basic read/skip methods. readAllBytes, readNBytes, skipNBytes and transferTo fell through to the generic InputStream defaults, which allocate 16 KiB scratch buffers in a loop and then concatenate — wasteful when the whole payload already sits in a byte[]. skip also advanced offset unconditionally via Math.addExact(offset, Math.toIntExact(n)), so a large n could throw ArithmeticException and offset could end up past eod.

Modification

• Override readAllBytes, readNBytes(int), readNBytes(byte[],int,int), skipNBytes and transferTo to operate directly on the backing array (one Arrays.copyOfRange / OutputStream.write each) — the same set of overrides java.io.ByteArrayInputStream has.
• Clamp skip to the remaining bytes so offset <= eod always holds and large skips cannot overflow; simplify available() and readLocal() accordingly.
• Simplify the (data, offset, length) constructor so offset/eod are clamped to the array using long arithmetic (no int overflow), and use Objects.checkFromIndexSize for the read(byte[],int,int) bounds check.
• Expand UnsynchronizedByteArrayInputStreamSpec to cover every method and edge case (EOF, zero-length reads, clamping, overflow, negative args).
• Add readAllBytes and transferTo cases to ByteString_asInputStream_Benchmark.

Result

readAllBytes/readNBytes/transferTo on ByteString.asInputStream copy the data once instead of chunking through temporary buffers. Observable behaviour is otherwise unchanged (negative skip still throws, as before).

Local JMH (-f 1 -wi 2 -i 3, short run so error bars are wide):

Benchmark	kb	before (ops/s)	after (ops/s)
single_bs_as_input_stream_read_all_bytes	10	201,138	502,768
single_bs_as_input_stream_read_all_bytes	1000	3,307	5,460
single_bs_as_input_stream_transfer_to	10	238,211	267,642
single_bs_as_input_stream_transfer_to	1000	2,883	2,747

transferTo is dominated by the ByteArrayOutputStream copy on the receiving side, so the difference there is within noise.

Tests

• sbt "actor-tests/testOnly org.apache.pekko.util.UnsynchronizedByteArrayInputStreamSpec org.apache.pekko.util.ByteStringSpec" — 226 passed
• sbt "actor/mimaReportBinaryIssues" — no issues (class is @InternalApi; only methods added)
• sbt "bench-jmh/Jmh/compile" and Jmh/run of the new benchmarks
• scalafmt on changed Scala files, sbt actor/javafmtAll on JDK 17
• git diff --check

References

None - follow-up to #2300 which introduced this class