fix(caching): only cache negative entries from authoritative listings

[nk412]

Description

Prevent empty windowed or paginated object listings from creating a negative stat-cache entry for an entire implicit-directory prefix.

fastStatBucket.insertListing previously treated every empty listing as evidence that the requested directory did not exist. That is not authoritative when the request uses StartOffset or a continuation token, or when the response contains a continuation token. In those cases, the listing represents only part of the namespace, and inserting a negative entry can overwrite a valid positive directory entry and cause transient ENOENT responses.

This change:

• passes the originating ListObjectsRequest to insertListing;
• limits listing-derived negative entries to unwindowed, complete listings;
• preserves negative caching for authoritative empty listings; and
• adds request-level coverage for start offsets and continuation tokens, plus a regression test using the real LRU/stat-cache implementation.

Link to the issue in case of a bug fix.

Fixes #5018

Testing details

1. Manual - Verified the deterministic regression fails on the unpatched implementation with gcs.NotFoundError: negative cache entry for dir/ and passes with this change. make build also passes.
2. Unit tests - Passed in golang:1.26.5 Docker:
   • go test ./internal/storage/caching -count=1
   • CGO_ENABLED=0 go test ./internal/storage/... -count=1
   • go test -race ./internal/storage/caching -count=1
   • go vet ./internal/storage/caching
   • CGO_ENABLED=0 go build ./...
3. Integration tests - Added TestIntegration_WindowedListingDoesNotPoisonExistingDirectory, which exercises fastStatBucket with the real LRU/stat cache and fake GCS backend. A live FUSE/GCS end-to-end test was not run.

Any backward incompatible change? If so, please explain.

N/A. The changed method is internal, and no exported API, configuration, or existing authoritative-listing behavior changes.

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[gemini-code-assist]

Code Review

This pull request modifies the caching behavior in fast_stat_bucket.go to prevent windowed or incomplete listings (such as those with a StartOffset or ContinuationToken) from triggering negative caching. This ensures that incomplete listings do not overwrite valid positive cache entries and cause incorrect ENOENT errors. Unit and integration tests have been added to verify this behavior. There are no review comments, so no additional feedback is provided.