fix: skip missing remote table metadata

[leno23]

Summary

• detect permanent remote metadata-missing errors before the download metadata retry loop keeps backing off
• warn and skip the affected table when its JSON metadata file is absent on remote storage
• preserve the existing optional .sql handling for incremental/embedded metadata and return non-not-found errors directly
• add a focused regression test for the not-found classifier patterns seen from common object stores

Tests

• go test ./pkg/backup -run TestIsRemoteMetadataNotFound -count=1
• go test ./pkg/backup -count=1

Closes #1379