fix(notes): delete legacy three-level fanout paths before writes

[jiangyq9]

What & why

notes_add_batch and notes_add_blob_batch rewrite notes into the canonical two-level fanout path, but the delete step only covered the flat path and the current two-level fanout path. Repositories that already contain an older three-level fanout note path can keep that stale note alongside the rewritten note.

That leaves two note entries for the same annotated object. git notes show may concatenate both note payloads, which breaks JSON deserialization and can make downstream stats treat commits as untracked.

Root cause

The note rewrite path removed:

• <object-sha>
• <aa>/<rest-of-object-sha>

but did not remove legacy three-level paths:

• <aa>/<bb>/<rest-of-object-sha>

Change

• Add a shared helper that resolves every supported storage path for a note object:
  • flat path
  • current two-level fanout path
  • legacy three-level fanout path
• Use that helper when resolving note blob OIDs so legacy notes are still discoverable.
• Delete all supported old note paths before writing the canonical two-level note path in both:
  • notes_add_batch
  • notes_add_blob_batch
• Add unit coverage for the note-path expansion helper.

Testing

• Not run in this environment.

Fixes #1517

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