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

• InMemoryPublisher.fulfillDemand() — Fix re-entrant deadlock: return immediately on re-entrant call instead of spinning; outer do-while re-checks demand after releasing for multi-threaded safety.
• InMemoryPublisher.request() — Validate n > 0 per Reactive Streams spec §3.9.

Testing

Added unit, integration, and TCK 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

[joviegas]

Can we scope this PR to just the root cause: the fix in InMemoryPublisher.fulfillDemand(), where a re-entrant request() call fails the sending compare-and-set and then spins forever instead of returning. That one change resolves the reported hang.

The other changes (FlexibleChecksummer ordering, contentLength propagation, the Content-Length/Transfer-Encoding fix in MakeAsyncHttpRequestStage) are independent fixes. Can we raise them as separate PRs so each is easy to review, test, and bisect if anything regresses?

[github-actions]

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