azure auth error handling

[shivv23]

Summary

This PR hardens Azure storage auth signing by replacing panic paths with typed errors and improving canonicalized resource handling.

Changes

• Updated AzureAuth::generate_headers to return Result<_, CloudError> instead of panicking.
• Replaced expect/unwrap with explicit CloudError::Auth mapping.
• Improved canonicalized query handling for Azure signed resources.
• Updated Azure blob client methods to propagate auth-generation errors.
• Added unit tests for:
  • missing AZURE_STORAGE_KEY
  • invalid base64 key
  • successful header generation with valid key

Why

• Prevents runtime panics in auth-critical paths.
• Gives callers actionable, typed errors.
• Improves robustness for real-world request query shapes.

Validation

• cargo check --lib