This PR fixes Azure blob upload verification by replacing the broken TransactionalValidation approach with a download-and-compare method.

Problem

The Azure SDK's TransactionalValidation with blob.TransferValidationTypeMD5() does not work correctly for UploadStream because it uploads files in chunks and validates MD5 against each chunk rather than the whole file.

Solution

• New verification approach: Calculate MD5 before upload, then download the uploaded file and recalculate MD5 to verify integrity
• Opt-in behavior: Verification is disabled by default to avoid performance overhead. Enable via VerifyUpload field or ?verify_upload=true query parameter in the storage URL
• Refactored MD5 helpers: md5FromStream is now the core function, with md5FromReader wrapping it for seekable readers

Changes

• az.go:

  • Added VerifyUpload field to Az struct
  • Parse ?verify_upload=true query parameter from URL
  • Implement download-and-verify logic with detailed error messages including hex digests
  • Remove unused blob import, add bytes for bytes.Equal

• request.go:

  • Refactored md5FromReader to use md5FromStream as core
  • Removed custom md5Equal in favor of bytes.Equal

• az_test.go:

  • Added TestNewAzFromUrl with comprehensive test cases for URL parsing including verify_upload parameter

Usage

// Via URL parameter
az, _ := NewAzFromUrl("az://myaccount.blob.core.windows.net/container?verify_upload=true")

// Or set directly
az := &Az{
    Account:      "myaccount.blob.core.windows.net",
    Container:    "container",
    VerifyUpload: true,
}