[Follow-up] TakeSnapshot: EnsureBucket + Core client wiring for snapshot upload

[coderabbitai]

Background

This issue tracks the follow-up implementation deferred from PR #65 (discussion: #65 (comment)).

service/external_snap.go의 TakeSnapshot 함수는 현재 Core 하위호환성 문제로 의도적으로 미완성 상태입니다.

Tasks

1. Idempotent EnsureBucket helper (in client/rustfs.go)

• Add an EnsureBucket(ctx, bucket) method on RustFSClient that calls CreateBucket and silently swallows BucketAlreadyOwnedByYou / BucketAlreadyExists errors.
• This prevents NoSuchBucket errors when Core later PUTs to the presigned URL.

2. Call EnsureBucket in TakeSnapshot (in service/external_snap.go)

• Before calling PresignPutObject, call rustfs.EnsureBucket(ctx, string(uuid)).

3. Wire up Core client call

• Pass the generated presignedURL to the Core API when CreateVM is updated to include presigned URL support (Story 2+).
• Replace the current // TODO comment with the actual Core client call.

When to implement

이 작업은 CreateVM에 presigned URL 지원이 추가되는 시점에 함께 개발되어야 합니다.

References

• PR Add rustfs #65: RustFS S3 Client Integration
• Linked issue: RustFS client initialization #64 (RustFS client initialization)

/cc @kwonkwonn