Tracking Copier API

# Summary

Track the remaining work for the `Copier` API proposed by RFC #7519.

# Completed

- [x] RFC: #7519
- [x] Phase 1: API skeleton and one-shot compatibility: #7527
- [x] Phase 2: S3 multipart copier: #7541
- [x] Phase 3: GCS rewrite copier: #7545
- [x] Phase 4: Azure Blob block copier: #7551

# Remaining tasks

## Phase 5: S3-compatible standalone services

- [ ] Evaluate whether `cos`, `oss`, and `obs` can share or adapt the existing S3 multipart copier design.
- [ ] Implement multipart copy for services that expose compatible upload-part-copy APIs.
- [ ] Add service-specific copy multi capabilities only after the protocol path is implemented.

## Phase 6: Monitor-based copy services

- [ ] Evaluate `onedrive` copy, which already starts an async server-side copy and polls a monitor URL internally.
- [ ] If useful, implement a stateful copier that stores the monitor URL and advances on `next()` instead of blocking inside `copy`.
- [ ] Do not expose provider monitor tokens in the public API.

## Phase 7: Remaining one-shot copy services

- [ ] Keep plain one-shot services as-is unless the backend exposes a real long-running or segmented server-side copy primitive.
- [ ] Do not add `copy_can_multi` for client-mediated local copies or provider APIs that only support one-shot copy.

# Constraints

- Keep backend continuation state internal.
- Do not emulate atomic destination conditions such as `if_not_exists` with `stat` plus unconditional copy.
- Prefer backend-native server-side copy primitives over client-mediated read plus write.
- Use `copy_can_multi` only when users can actively request segmented server-side copy through `chunk`.

# References

- RFC PR: #7519

Start with RFC #7519 and the existing S3 multipart copier design, then review the completed phases listed in this issue. Compare the remaining services against their server-side copy capabilities and the constraints around copy_can_multi, chunk, and next(). Done means the applicable copier paths and capability behavior are implemented without exposing monitor state or emulating destination conditions.

Written by the indexing model from the issue text.