Fix authentication and error handling for cloud registries

[vsbuffalo]

Thank you again for oras-py! I have found a few more issues and have included a PR below. I will also try to submit a PR soon for #217 (I was able to address that downstream so less pressing, but I will submit PR soon).

Problems Fixed

1. Authentication headers not being applied consistently

The do_request() method only adds authentication headers when very specific conditions are met:

if isinstance(self.auth, oras.auth.TokenAuth) and self.auth.token is not None:
    headers.update(self.auth.get_auth_header())

This restrictive check causes authentication to fail in several scenarios:

• When using custom auth backends that aren't exactly TokenAuth instances
• When auth backends are wrapped or proxied
• When using OAuth2 bearer tokens with cloud registries (Docker Hub, ECR, GCR, ACR, Artifact Registry, etc.)

2. JSONDecodeError on non-JSON registry responses

When registries return non-JSON responses (e.g., HTML error pages from web portals), the code attempts to parse them with r.json(), resulting in confusing errors:

json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)

This commonly occurs with any cloud registry when:

• Authentication failures return HTML error pages instead of JSON
• Hitting web portals/management consoles instead of registry API endpoints
• Network issues or misconfigurations redirect to error pages

Solutions

Commit 1: Use duck typing for auth header application

Relaxes the authentication check to use duck typing, which is more Pythonic and flexible:

if self.auth and hasattr(self.auth, 'get_auth_header'):
    auth_headers = self.auth.get_auth_header()
    if auth_headers:  # Only update if we got actual headers
        headers.update(auth_headers)

Note: In the future, defining an auth interface with a Python Protocol could make the contract more explicit, but the current duck typing approach works well and is less opinionated of a design choice.

Commit 2: Improve error handling for non-JSON responses

• Replace r.json() calls in error handling with descriptive HTTP status messages
• Add fallback for capitalized Location header (some registries use "Location" instead of "location")
• Provide actionable error messages for debugging authentication issues

Benefits

1. Duck typing: More Pythonic - if an object has get_auth_header(), we can use it
2. Works with all auth backends: Not limited to just TokenAuth class
3. Better error messages: Users see actual problems (auth, permissions, connectivity) instead of JSON parsing errors
4. Robust header handling: Works with registries that use different header capitalizations
5. Backward compatible: All existing auth backends already implement required methods

Testing

All unit tests pass. I have tested with OAuth2 bearer token authentication (Azure Container Registry):

• Before: Authentication headers not consistently applied, push operations fail with confusing JSON decode errors
• After: Headers properly added to all requests, clear error messages, push operations succeed

Verified with cloud registry implementations including Azure Container Registry.

Compatibility

Both changes are fully backward compatible:

• All existing auth backends (BasicAuth, TokenAuth) already have get_auth_header() method
• Error handling improvements only change error message content, not behavior
• The changes only extend support to additional auth backend implementations and registry types