• I added CHANGELOG entry for this change.
• Change is not relevant to the end user.

Changes

Fixes #5962.

• Add a shared block marker-gathering helper for metadata filters that scan marker JSON files.
• Use it from the no-compact and no-downsample marker filters while keeping their typed accessors and error wrapping intact.
• Add focused tests for the shared helper and both component filter wrappers.

Verification

• go test -tags slicelabels ./pkg/block ./pkg/compact ./pkg/compact/downsample -run 'TestGatherMarkedBlocks|TestGatherNoCompactionMarkFilter|TestGatherNoDownsampleMarkFilter' -count=1
• THANOS_TEST_OBJSTORE_SKIP=GCS,S3,AZURE,SWIFT,COS,ALIYUNOSS,BOS,OCI,OBS go test -tags slicelabels ./pkg/block ./pkg/compact ./pkg/compact/downsample -count=1