cleanup: return NotFound if blob stored w/o chunking

[tyler-french]

This PR cleans up the implementation to adhere to the improved spec, and replaces Unimplemented with a more accurate NotFound error.

https://github.com/bazelbuild/remote-apis/blob/main/build/bazel/remote/execution/v2/remote_execution.proto#L494-L496

  // * `NOT_FOUND`: The requested blob is not present in the CAS, OR there is no
  //   split information available for the blob, OR at least one chunk needed to
  //   reconstruct the blob is missing from the CAS.

bazelbuild/remote-apis@080cf12 (bazelbuild/remote-apis#353) clarified that the SplitBlob server can return NOT_FOUND if the blob exists but is stored without chunking.

[Copilot]

Pull request overview

This PR updates the implementation to align with a clarification in the Remote Execution API specification regarding the SplitBlob RPC. According to the updated spec, SplitBlob should return NOT_FOUND not only when a blob doesn't exist, but also when a blob exists but was stored without chunking metadata. The PR simplifies the code by removing logic that tried to distinguish between these cases and return different error codes.

Changes:

• Removed comment explaining the old behavior of returning Unimplemented for blobs stored without chunking
• Simplified error handling in LoadManifest to always return the sanitized error without checking if the blob exists
• Removed unit test that verified Unimplemented error was returned for blobs that exist but lack chunking metadata

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
server/remote_cache/content_addressable_storage_server/content_addressable_storage_server.go	Removed outdated TODO comment explaining the old behavior
server/remote_cache/chunking/chunking.go	Simplified LoadManifest error handling to return NOT_FOUND in all cases, removed errors import
server/remote_cache/chunking/chunking_test.go	Removed test that verified old behavior of returning Unimplemented for blobs without chunking

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[maggie-lou]

This change LGTM, but is there a world where we'd want the server to return the blob even if it is stored without chunking? Or is that unlikely to happen anyway once we're fully migrated to the CDC cache?

[tyler-french]

This change LGTM, but is there a world where we'd want the server to return the blob even if it is stored without chunking? Or is that unlikely to happen anyway once we're fully migrated to the CDC cache?

Yes, I'd like to experiment with on-demand chunking. Maybe even using an executor to split the blob. Added to https://github.com/buildbuddy-io/buildbuddy-internal/issues/6426