fix: count CommonPrefixes in POSIX ListObjectsV2 responses

[acerrah]

Problem

The POSIX backend calculated ListObjectsV2.KeyCount using only the
number of returned objects:

len(results.Objects)

When a delimiter listing returned only CommonPrefixes, the response
contained KeyCount=0 even though prefixes were present. This caused
Docker Registry garbage collection to fail with:
Path not found: /docker/registry/v2/repositories

Solution

Calculate KeyCount using both returned objects and common prefixes:

len(results.Objects) + len(results.CommonPrefixes)

This matches AWS S3 semantics and the existing Azure backend behavior.

Testing

• go test ./backend/posix ./backend passed.
• go test ./... passed.

AI Assistance

AI was used during the investigation and writing process to inspect the relevant code paths, identify the KeyCount calculation issue, and help prepare this pull request description. The final code change and test
results were reviewed manually.

[versity-github]

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