x-pack/filebeat/input/awss3: exclude backup-prefixed keys from polling

[efd6]

Proposed commit message

x-pack/filebeat/input/awss3: exclude backup-prefixed keys from polling

When same-bucket backup is configured and bucket_list_prefix is empty
(the default), the S3 listing returns backup objects alongside source
objects. Because each backup has a new key, ETag, and LastModified, the
state registry never recognises it as already processed. This causes
infinite reprocessing: each poll cycle processes its own prior backup
output and creates a deeper copy.

Exclude keys that start with the configured backup prefix from the
listing results when the backup destination is the same bucket and the
prefix falls within the listing scope. This applies to both the v1
poller (s3_input.go) and v2 poller (v2_polling.go).

Fixes #51852

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works. Where relevant, I have used the stresstest.sh script to run them under stress conditions and race detector to verify their stability.
• I have added an entry in ./changelog/fragments using the changelog tool.

Disruptive User Impact

How to test this PR locally

Related issues

• Fixes [bug-hunter] AWS S3 same-bucket backups are recursively reprocessed #51852

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