Fix listing bounds in S3 and GS stores

[sduchesneau]

Bugs

Both were caught by new shared storetests cases, verified by reverting the source and watching each one fail on exactly the store that had it (MinIO / fake-gcs).

• S3 WalkFromTo compared its exclusive end point against a prefix-stripped name while the walked names are store-relative. With a non-empty prefix the bound never matched, so the walk yielded keys past it and paged to the end of the prefix.
• GS WalkFromTo built the listing's end offset with filepath.Join, which ate the trailing /. Walking up to alpha/ dropped the object named alpha, which sorts before the bound.

Same results, less work

• ListFoldersFromTo on S3 and Azure returns at the first folder >= exclusiveTo instead of listing the rest of the prefix to discard it. Azure has no server-side bound, so this is its only lever.
• S3 ListFoldersFromTo pushes its lower bound down whole, minus the trailing / that makes StartAfter exclusive. It used to drop the bound's last character, and push nothing at all when a single character was left.
• S3 WalkFromTo starts at the key right before the starting point (helloworld.html → helloworld.htmk) instead of the stem (helloworld.htm), which had the service send every key in between for the walk to filter out. On fixed-width keys such as block numbers that window held every key sharing the stem; it now holds none. New keyBefore() decrements the last rune, so the marker stays valid UTF-8, and falls back to dropping it (control characters, invalid bytes, the surrogate boundary).

Tests

• storetests: TestWalkFromTo_WithPrefix, TestWalkFromTo_FolderEndPoint, TestListFoldersFromTo_SingleCharacterBounds — the suite only ever walked ranges with an empty prefix and never used a slash-terminated end point, which is how both bugs got in.
• s3store_mock_test.go: a recording RoundTripper serving canned pages, asserting the start-after/prefix/delimiter each bound shape produces and proving early termination by round-trip count — a real backend can't show you that. Plus a keyBefore table with a sorts-before invariant.
• gsstore_test.go: TestGSStore_calculateEndOffset.

Green against MinIO, Ceph, fake-gcs and Azurite (docker compose up -d), plus local/memory/mock.

Unrelated, pre-existing: azure_test.go TestAzureStoreWriteObject gates on AZURE_STORAGE_KEY but hardcodes account streamingfasttest01/container myblobs, so pointing it at Azurite 404s. Left alone.

🤖 Generated with Claude Code

https://claude.ai/code/session_01ESMxv9zsuiAXikNs1wwYup

[sduchesneau]

LGTM (claude/gpt sol wrote that)

[UlysseCorbeil]

Nothing jumps out, lgtm