[codex] Add Upload missing-file regression test

[aftersnow]

Summary

• add a focused backend regression test for Upload when the input file is missing
• assert Upload preserves the getProcessor stat failure introduced by fix: use InferFileType in getProcessor for unrecognized file extensions #499
• cover a recent behavior change that previously only had helper-level tests

Why

#499 changed Upload to surface getProcessor errors, but the package only tested getProcessor directly. This leaves the public Upload entrypoint unguarded against regressing back to the older generic error path.

Validation

• go test ./pkg/backend -run TestUploadMissingFileReturnsProcessorStatError -count=1
• go test ./pkg/backend -count=1
• red state on 1c9e7ff^: go test ./pkg/backend -run TestUploadMissingFileReturnsProcessorStatError -count=1

[gemini-code-assist]

Code Review

This pull request adds a new unit test in pkg/backend/upload_test.go to verify that the Upload function correctly handles missing files by returning an appropriate error message. I have no feedback to provide.