Fix TransferListener callbacks for unknown content length uploads

Motivation and Context

TransferListener callbacks like transferInitiated and transferComplete were not being called for single part, unknown-content length uploads.

Modifications

• Always call progressListener.subscriberOnComplete() for successful single part uploads.

Why not set REPORT_PROGRESS_IN_SINGLE_CHUNK for this case instead?
For unknown content length we don't know ahead of time whether this will be a single chunk or multipart (this decision happens deep in UploadWithUnknownContentLengthHelper after the transfer manager has already set REPORT_PROGRESS_IN_SINGLE_CHUNK.

Testing

Added new unit tests + manual reproduction of issue.

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
• 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.

License

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