Preserve contentLength for InMemoryPublisher

[davidh44]

Motivation and Context

Follow up to #7129

After fixing the InMemoryPublisher deadlock, InMemoryPublisher still reports no contentLength() because it only implements bare Publisher<ByteBuffer>. This means the CRT HTTP client receives getLength()=0 ("unknown length"). Additionally, FlexibleChecksummer has a theoretical race condition where checksum headers could be set after signing due to a detached thenRun side-effect.

When InMemoryPublisher correctly reports its content length, a latent bug in MakeAsyncHttpRequestStage is exposed: it adds Content-Length even when Transfer-Encoding: chunked is already present, violating RFC 7230 §3.3.2.

These are not customer-facing bugs today: CRT handles getLength()=0 correctly and the race condition only manifests with async body sources. This PR is defensive hardening to prevent future issues.

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 — 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.

Testing

Added unit tests, updated TCK test

Screenshots (if appropriate)

Types of changes

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

Checklist

• I have read the CONTRIBUTING document
• Local run of mvn install succeeds
• My code follows the code style of this project
• My change requires a change to the Javadoc documentation
• I have updated the Javadoc documentation accordingly
• I have added tests to cover my changes
• All new and existing tests passed
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• My change is to implement 1.11 parity feature and I have updated LaunchChangelog

License

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