fix: skip CRT Content-Length when chunked

[zoewangg]

Motivation and Context

When the AWS CRT-based async HTTP client sends a request with a body of unknown
length, the SDK marshals it with Transfer-Encoding: chunked and no
Content-Length. However, the CRT request adapter also synthesized a
Content-Length from the request body publisher's reported length whenever one
was available. This could produce a request carrying both
Content-Length and Transfer-Encoding: chunked.

Per RFC 7230 (section 3.3.2/3.3.3) a sender must not emit Content-Length
together with Transfer-Encoding. The underlying CRT layer enforces this and
rejects such a request with AWS_ERROR_HTTP_INVALID_HEADER_FIELD
("Both Content-Length and Transfer-Encoding are set. Only one may be used").

This is RFC 7230 compliance hardening. It aligns the CRT client with the Netty
client, which never synthesizes a Content-Length from the publisher length for
a chunked request. Note: for the operations that actually use request chunked
transfer encoding (unsigned-payload operations) over their normal HTTPS
endpoints, the payload is unsigned and no concrete publisher length is
introduced, so real callers did not hit the conflict; this is defense-in-depth,
not a fix for a runtime hang.

Modifications

CrtRequestAdapter.createAsyncHttpHeaderList (aws-crt-client): no longer adds
the Content-Length header when the request already carries a
Transfer-Encoding header. Uses the existing Header.CONTENT_LENGTH /
Header.TRANSFER_ENCODING constants and the existing firstMatchingHeader
lookup style. The sync createHttpHeaderList was left unchanged: it never
synthesizes Content-Length (it only copies headers already on the request),
so it cannot produce the conflict.

Testing

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](https://github.com/aws/aws-sdk-java-v2/blob/master/CONTRIBUTING.md) 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
• I have added a changelog entry. Adding a new entry must be accomplished by running the scripts/new-change script and following the instructions. Commit the new file created by the script in .changes/next-release with your changes.
• My change is to implement 1.11 parity feature and I have updated [LaunchChangelog](https://github.com/aws/aws-sdk-java-v2/blob/master/docs/LaunchChangelog.md)

License

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

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