Fix PutObject with CRT client chunkedEncodingEnabled(false)

[davidh44]

Motivation and Context

S3 PutObject hangs indefinitely when using AwsCrtAsyncHttpClient with chunkedEncodingEnabled(false). Bug introduced in 2.30.0 when requestChecksumCalculation defaulted to WHEN_SUPPORTED, which triggers automatic checksum computation for PutObject. With chunked encoding disabled, the SDK buffers the payload into InMemoryPublisher to compute a header-based checksum. CRT's subscriber calls request(1) synchronously from within onNext() (legal per Reactive Streams spec), causing a re-entrant deadlock in InMemoryPublisher.fulfillDemand().

Modifications

Checksum computation (http-auth-aws)

• FlexibleChecksummer — Replace detached thenRun with thenApply to guarantee checksum headers are set before signing proceeds. Theoretical race condition fix — prevents issues if the body source delivers asynchronously.
• ChecksumSubscriber — Compute total buffered bytes in onComplete() and pass to InMemoryPublisher.
• InMemoryPublisher — Fix re-entrant deadlock in fulfillDemand(): return immediately on re-entrant call instead of spinning; outer do-while re-checks demand after releasing for multi-threaded safety. Also implement SdkHttpContentPublisher to report contentLength().

Signing (sdk-core)

• AsyncSigningStage — Preserve contentLength() when wrapping SdkHttpContentPublisher payload into AsyncRequestBody, instead of always using fromPublisher() which loses it.

HTTP request construction (sdk-core)

• MakeAsyncHttpRequestStage — Don't set Content-Length if Transfer-Encoding is already present. These are mutually exclusive per RFC 7230 §3.3.2. Pre-existing latent bug exposed by InMemoryPublisher now correctly reporting content length.

Testing

Added unit and integration tests

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

License

• I confirm that this pull request can be released under the Apache 2 license