Provide an opt-in zero-copy response body view - #2322

Mockito 4 cannot invoke interface default methods through
CALLS_REAL_METHODS on JDK 21 and newer. Invoke the Response default
method explicitly so delegation remains covered on every supported JDK.

Refs AsyncHttpClient#2321

Codex on behalf of Pavel Ptashyts

Co-Authored-By: OpenAI Codex <noreply@openai.com>

Review feedback on the response body view.

The contract was written as though the sharing were the response's to
give. It is not. Whether a lone body part exists to share from is a fact
about how the body arrived - how the origin chunked it, whether a proxy
re-chunked it, whether it was compressed - so the same body from the
same server can be shared on one response and copied on the next, and
the caller sees no difference. And where an array is shared it is not
the response's alone: it is reachable from the part handed to
onBodyPartReceived, it is what a TransferCompletionHandler gives each
TransferListener, and it is what getResponseBodyAsByteBuf wraps. A write
through any of those changes what this returns, and a write through this
changes what they see.

None of that can be designed away while the method returns a byte array,
so the javadoc says it, and says the identity between calls is not
guaranteed either. It no longer promises anything on behalf of
getResponseBodyAsBytes, which is an interface method whose own contract
guarantees no independent array. It records that leaving the default in
place while implementing getResponseBodyAsBytes in terms of this method
makes the two call each other, which is the first thing an implementor
would otherwise write.

getResponseBody(Charset) goes back through the private helper rather
than the new public method, so overriding the view cannot change what a
response's text says as well; the helper keeps the note about multi-byte
characters straddling a part boundary, which is the only place that
reason is written down, and now says that the array does leave the
client rather than claiming it does not.

An empty body returns a shared empty array rather than walking the
aggregating path to allocate one and a buffer to wrap it. Nothing can be
written through a zero-length array.

The tests asserted identity where they meant content, so an
implementation that shared a corrupted array satisfied them, and one
took its expectation from the array it had handed to the part, which
would have stopped being an oracle the moment the part stopped copying.
The lazy coverage this branch added is dropped: a Response built from
lazy parts holds buffers at refCnt 0 on a real request, since
channelRead releases in a finally and LazyResponseBodyPart never
retains, and a test that keeps one alive by hand signs off on a mode
that does not work. That branch is still covered through
getResponseBody(Charset), which shares the helper.

Claude Code on behalf of Pavel Ptashyts

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>