Add bufferBeforeSend opt-in for reliable multipart upload retry with slow streaming sources

[alextwoods]

Add fullBufferingEnabled opt-in for reliable multipart upload retry with slow streaming sources

Motivation and Context

The S3 multipart upload pipeline has a structural defect when using BufferedSplittableAsyncRequestBody with slow streaming sources (e.g., SFTP). In the known-content-length path of SplittingPublisher, each part body is emitted downstream (opening the HTTP connection) immediately upon initialization — before the part data is fully buffered. If the server returns a retryable error mid-stream, the RetryableSubAsyncRequestBody retry buffer is null because complete() has not yet been called, resulting in a NonRetryableException.

This change adds an opt-in fullBufferingEnabled option that instructs SplittingPublisher to defer sending each part downstream until it is fully buffered — matching the behavior already used for the unknown-content-length path. The existing default behavior (immediate send) remains unchanged.

Modifications

SplittingPublisher.java (internal)

• Added fullBufferingEnabled field and corresponding Builder.fullBufferingEnabled(boolean) method.
• Changed initializeNextDownstreamBody(): if (contentLengthKnown) → if (contentLengthKnown && !fullBufferingEnabled) to defer sending when full buffering is enabled.
• Changed completeCurrentBody(): if (upstreamSize == null && ...) → if ((upstreamSize == null || fullBufferingEnabled) && ...) to send after complete when full buffering is enabled.

BufferedSplittableAsyncRequestBody.java (public API)

• Added private final boolean fullBufferingEnabled field.
• Added new factory method: create(AsyncRequestBody delegate, boolean fullBufferingEnabled) with Javadoc.
• Updated splitCloseable() to pass fullBufferingEnabled to SplittingPublisher.Builder.
• Existing create(AsyncRequestBody) passes false, preserving backward compatibility.

Testing

Added new tests.

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
• 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

License

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