[samgbafa]

Linear: TC-1354

Problem

S3BlockStore::remove called GetObjectAttributes without an attribute list. S3 requires x-amz-object-attributes, and MinIO rejects the call with InvalidArgument: Invalid attribute name specified., so every S3 block delete returned an error.

No code on main or in the released node calls remove today (kv/del is logical and leaves blobs in place), so this does not affect production now. It matters as soon as something calls remove, such as #336 or a later cleanup.

Change

• remove now gets the size and checks existence with HeadObject, then deletes. A missing object still returns Ok(None). The space size drops by the object's stored length.
• The object_size and delete helpers match feat(TC-887): seal KV blocks in the S3 store (S3c, stacked on #333) #336's, so the TC-887 stack rebases with no logic conflict.
• New tinycloud-node-server/tests/s3_block_store.rs runs against the at-rest harness's native MinIO. It is #[ignore]d by default, and the at-rest harness CI job runs it with the harness env loaded.

Evidence (local, native MinIO RELEASE.2025-09-07T16-13-09Z, no Docker)

• RED on main 951d2f5: remove_deletes_the_block_and_releases_its_size fails with InvalidArgument: Invalid attribute name specified.
• GREEN with this change. The test covers four things: removing a block, a sibling block surviving, the size dropping by exactly the removed length, and a second remove (or a remove of a never-stored hash) returning None without changing the size.
• cargo test -p tinycloud-node --lib storage:: passes, cargo fmt --all -- --check is clean, and RUSTFLAGS=-Dwarnings cargo clippy -p tinycloud-node --all-targets is clean.

[samgbafa]

APPROVE: see TC-1354 (head e8a176abaf7075e2ac17fbeba6b82a59604dcd0e)