Provide an opt-in zero-copy response body view

[pavel-ptashyts]

Summary

• add Response#getResponseBodyAsBytesView() as a compatible default method
  with an explicit possibly-shared, read-only contract
• return the existing byte array for a single eager Netty body part
• use the body part accessor for lazy/direct/sliced buffers, avoiding a second
  aggregate copy without exposing ByteBuf storage
• preserve defensive-copy behavior for getResponseBodyAsBytes(),
  getResponseBodyAsByteBuffer(), and getResponseBodyAsStream()
• reuse the new accessor for string decoding so the fast path from Decode a lone response body part in place #2303 stays
  centralized

Closes #2321.

Compatibility and safety

The default implementation delegates to getResponseBodyAsBytes(), so existing
third-party Response implementations keep their current behavior. Netty uses
the optimized path only for exactly one body part. Empty and multipart bodies
continue through the existing aggregation path.

Tests cover eager identity and repeated access, defensive-copy isolation,
heap and direct lazy slices, reader/writer indices, reference counts, empty
bodies, multipart ordering and split UTF-8 characters, stream isolation, and
default-method delegation.

Verification

• mvnw.cmd -B -ntp -pl client -Dtest=NettyAsyncResponseTest test
• mvnw.cmd clean verify
  • Amazon Corretto 11.0.32.1
  • 1,489 tests; 0 failures; 0 errors; 26 skipped
  • Error Prone, NullAway, Javadocs, GPG signing, and Revapi passed

Allocation benchmark

JMH 1.37, Corretto 11.0.32.1, one fork, three 500 ms warmups, five
500 ms measurements, gc profiler. Values are allocated bytes per operation,
rounded to whole bytes; zero means below the profiler's resolution.

Body shape	getResponseBodyAsBytes()	getResponseBodyAsBytesView()
Empty	16 B/op	16 B/op
1 eager part, 512 B	576 B/op	0 B/op
1 eager part, 4 KiB	4,160 B/op	0 B/op
1 eager part, 16 KiB	16,448 B/op	0 B/op
1 eager part, 128 KiB	131,136 B/op	0 B/op
1 lazy direct part, 4 KiB	8,272 B/op	4,112 B/op
1 lazy direct part, 16 KiB	32,848 B/op	16,400 B/op
2 eager parts, 4 KiB each	8,256 B/op	8,208 B/op
8 eager parts, 4 KiB each	32,832 B/op	32,832 B/op

The eager single-part view measured about 2.3 ns/op regardless of body size,
versus 322 ns/op for 4 KiB and 11.0 us/op for 128 KiB defensive copies. Lazy
single-part allocation is halved as expected. Multipart bodies retain the
payload-sized aggregation allocation.

Codex on behalf of Pavel Ptashyts

[pavel-ptashyts]

Round 1 addressed. Two of the comments are about the contract rather than the code, and they are right that neither can be designed away while the method returns a byte[]: whether a lone part exists to share from is decided by the chunking, and where an array is shared it is also reachable from onBodyPartReceived, from a TransferCompletionHandler's listeners and from getResponseBodyAsByteBuf(). So the javadoc states both instead of glossing them, and states that array identity between calls is not guaranteed either.

That leaves an accessor whose honest description is "will not copy when it does not have to, and will not tell you when that is". If that is not worth an interface method, I would rather you said so than have me argue it - it closes cleanly, and getResponseBodyAsByteBuf() already covers the multi-part case zero-copy for anyone who can take a buffer.

The rest:

• getResponseBody(Charset) goes back through the private helper, so overriding the view cannot change a response's text as well. The helper keeps Decode a lone response body part in place #2303's note about multi-byte characters straddling a part boundary, and now says the array does leave the client rather than claiming it does not.
• The javadoc no longer promises anything on behalf of getResponseBodyAsBytes(), and records the recursion an implementor would otherwise write - narrowed to the case that actually recurses, which is leaving the default in place.
• @implSpec is a paragraph rather than a tag: it is not configured for the javadoc plugin here and the build fails on unknown tag. Offered separately.
• Empty bodies short-circuit to a shared empty array.
• Tests: content assertions where there were only identity ones, expected.clone() so the oracle is not the fixture, and the LAZY coverage dropped rather than signing off on a mode that cannot occur on a real request.

./mvnw clean verify green. JDK 17 locally, no 11 on this machine.