Implement resumable downloads with HTTP Range requests for model-cli pull

[Copilot]

Adds resumable download support to handle interrupted model pulls. On retry, the system checks for .incomplete files, verifies server Range request support via Accept-Ranges header, and resumes from the last byte position. Falls back to full download if Range not supported. Includes retry logic (3 attempts, exponential backoff: 1s→2s→4s).

Changes

go-containerregistry fetcher (pkg/go-containerregistry/pkg/v1/remote/)

• Added fetchBlobWithOffset(offset) to send Range: bytes={offset}- headers
• Added supportsRangeRequests() to detect server capability via HEAD request
• Skip hash verification for partial content (206 responses) since incomplete data can't be validated
• Extended remoteLayer with CompressedWithOffset() and SupportsRangeRequests() methods
• Propagated resumable methods through MountableLayer wrapper

Store resumable orchestration (pkg/distribution/internal/store/blobs.go)

• Added ResumableBlob interface and writeBlobResumable() with:
  • .incomplete file detection and size check
  • Server Range support verification before resume attempt
  • Offset-based download continuation or full redownload fallback
  • Exponential backoff retry (max 3 attempts)
• Modified writeLayer() to detect and use resumable layers via type assertion

Example Flow

// Store detects incomplete file
if info, err := os.Stat(incompletePath); err == nil {
    offset = info.Size()
    
    // Check server capability
    if supportsRange, _ := layer.SupportsRangeRequests(); supportsRange {
        // Resume from offset
        rc, _ := layer.CompressedWithOffset(offset)
    } else {
        // Full download fallback
        os.Remove(incompletePath)
    }
}

Maintains full backward compatibility with existing non-resumable download paths.

Original prompt

Implement resumable downloads for "model-cli pull", implement in pkg/go-containerregistry/ , if all processes die it should be able to:

On next attempt, the system:
- Checks for .incomplete file and gets its size
- Check if the server supports range requests
- If it does, sends the HTTP Range request: Range: bytes={size}-, appends new data to existing file, else download from scratch as the server doesn't support Range requests.
- Retries up to 3 times with exponential backoff

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