Preserve existing getObject files

[davidh44]

Motivation and Context

FileAsyncResponseTransformer deleted the destination whenever FailureBehavior.DELETE was configured, even when the current request attempt failed before opening the file. With CREATE_NEW, this could delete a pre-existing file and allow a retry to silently replace it.

Modifications

This change resets file-channel state for each attempt, clears it during failure cleanup, and deletes the destination only when the current attempt successfully opened it. Existing files are preserved when opening fails or the request fails before writing. Partial output is still deleted after a successful open when FailureBehavior.DELETE is configured.

Behavior change: an async download with CREATE_NEW onto an existing destination now surfaces
FileAlreadyExistsException instead of deleting the file and letting the retry overwrite it.

Callers that intentionally require replacement behavior should use getObject(request, AsyncResponseTransformer.toFile(path, FileTransformerConfiguration.defaultCreateOrReplaceExisting())).

Testing

Added unit tests

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.
• 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

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