Split seekable files from non-seekable files

[djeebus]

This allows us to cache, compress, etc.

• Add a new OpenSeekableObject method to the storage provider that is used to read seekable files
• Remove the ReadAt method from the ObjectProvider interface (only used by SeekableObjectProvider).
• Remove the Delete method from the ObjectProvider interface (never used).
• Add a SeekableObjectProvider interface.
• Add new ObjectType and SeekableObjectType types that defines which file type is being passed in, in order to help the compiler prevent mistakes

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Note

Split storage API into typed seekable and non-seekable object interfaces with OpenObject/OpenSeekableObject, updating providers, caching, mocks, tests, and orchestrator code to use explicit object types.

• Storage (core API):
  • Introduce ObjectType and SeekableObjectType enums and new interfaces: ObjectProvider (non-seekable) and SeekableObjectProvider (seekable).
  • StorageProvider now exposes OpenObject(ctx, path, objectType) and OpenSeekableObject(ctx, path, seekableObjectType).
  • Add typed constants: MemfileObjectType, RootFSObjectType, MemfileHeaderObjectType, SnapfileObjectType, MetadataObjectType, LayerObjectType, LayerMetadataObjectType.
• Providers:
  • AWS/GCP/FS providers implement both new interfaces; add Exists on non-seekable objects; helper ignoreNotExists (AWS) and typed open methods.
• Caching:
  • Split cache into CachedObjectProvider (full-file) and CachedSeekableObjectProvider (chunked ReadAt/Size); reorganize cache code into storage_cache_object.go and storage_cache_seekable.go.
  • CachedProvider updated to wrap both OpenObject and OpenSeekableObject.
• Orchestrator:
  • Switch reads of data files to OpenSeekableObject with storage.MemfileObjectType/storage.RootFSObjectType and headers/metadata to OpenObject with typed ObjectType.
  • Update storage diff/template build paths to carry and use object type; add helper mappers for diff/file types to object types.
  • Server layer upload now checks obj.Exists(ctx) instead of Size.
• Mocks/Tests/Config:
  • Replace StorageObjectProvider mocks with ObjectProvider and add SeekableObjectProvider mocks; update usages.
  • Adjust tests to new interfaces and semantics (e.g., Exists and delete via provider).
  • Update .mockery.yaml to generate new mocks and filenames.

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