sandbox-image: complete with dataplane-signed URI

[sgirones]

Summary

Sandbox image builds now get their rootfs upload URLs from the dataplane instead of from platform-api. When /prepare returns snapshotRelPath, the CLI asks the builder sandbox's proxy to call dataplane POST /api/v1/blob/sign; dataplane composes the final object URI from its regional config, signs the multipart upload, and returns that canonical uri with the upload spec.

The CLI treats dataplane's signed uri as the source of truth. It writes that URI into spec.snapshotUri so the in-sandbox builder keeps the existing input contract, keeps the same URI in CLI state, and sends it directly to /complete. Builder metadata still supplies computed fields like snapshot_size_bytes, snapshot_format_version, and rootfs_disk_bytes; if metadata includes snapshot_uri, the CLI checks that it matches the dataplane URI.

The legacy /prepare shape is unchanged. If snapshotRelPath is absent, the prepared spec already contains upload, and completion continues to use builder metadata with the prepared snapshotUri fallback.

Notes For Review

• snapshotRelPath is only used to request dataplane signing. Completion is keyed off the saved signed URI, not the rel path.
• The new upload response must include uri; missing uri fails before the builder runs.
• The upload block remains mostly opaque JSON in the passthrough spec. The CLI only reads uri.
• Diff builds still sign parent manifests as full-URI SingleGet requests.

Test Plan

• cargo +nightly fmt --check
• git diff --check
• cargo test -p tensorlake sandbox_images
• Manual smoke against legacy platform-api response with embedded upload.
• Manual smoke against new platform-api + sandbox-proxy sign_blob.

Related PRs

• platform-api: https://github.com/tensorlakeai/platform-api/pull/530
• compute-engine-internal: https://github.com/tensorlakeai/compute-engine-internal/pull/1039
• tensorlake: sandbox-image: complete with dataplane-signed URI #698

Flow

CLI -> platform-api /prepare
  <- snapshotRelPath

CLI -> sandbox-proxy /api/v1/blob/sign { rel_path, multipart_put }
  -> dataplane validates namespace, composes final URI, signs upload
  <- { uri, uploadId, partUrls, completeUrl, abortUrl }

CLI writes uri into spec.snapshotUri
CLI runs the in-sandbox builder
CLI completes with the saved dataplane-signed uri

For parent snapshot reads, the CLI uses { uri, op: SingleGet } because the parent manifest already has a full stored URI.