Core: Delete orphaned merged DV files on cache invalidation and commit failure

[wangyum]

What

MergingSnapshotProducer.mergeDVs() writes a Puffin file each time duplicate DVs are merged. When the delete manifest cache is invalidated (by a subsequent addDeletes call) or the commit fails, only the manifest files were deleted — the Puffin files were left orphaned on disk.

Why

After #15006, mergeDVs() writes a Puffin file (e.g. merged-dvs-<snapshotId>-<n>.puffin) and caches the resulting delete manifests. Two cleanup paths missed the Puffin files:

1. Cache invalidation (newDeleteFilesAsManifests): deleted the cached manifests but not the Puffin files they referenced.
2. Commit failure (cleanUncommittedAppends): deleted uncommitted delete manifests but not their Puffin files.

The orphaned files can only be removed by remove_orphan_files. No data corruption — read correctness is preserved.

How

Track merged DV file locations in a cachedMergedDVLocations field (paralleling cachedNewDeleteManifests) and delete them in both cleanup paths. The location is resolved through the FileIO so its scheme matches DeleteFile.location() (e.g. stripping a file: URI scheme).

Tests

• testMergedDVPuffinFileCleanedUpOnCacheInvalidation — verifies the Puffin file is deleted when a subsequent addDeletes invalidates the cache, and the committed snapshot has a correctly merged DV.
• testMergedDVPuffinFileCleanedUpOnCommitFailure — verifies the Puffin file is deleted when the commit fails (via injected failCommits).

Closes #17307

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AI Disclosure

• Model: GLM 5.2
• Platform/Tool: opencode
• Human Oversight: fully reviewed
• Prompt Summary: Fix orphaned merged DV Puffin file on cache invalidation and commit failure, with tests covering both paths.

[wangyum]

cc @amogh-jahagirdar @rdblue