v0.4.0

Breaking Changes:

• Provider.SetHeaders signature changed from SetHeaders(req *http.Request) to SetHeaders(ctx context.Context, req *http.Request) error

Added:

• pkg/identities package for managed identity token sources
  • AzureTokenSource type wrapping azidentity.ManagedIdentityCredential
  • NewAzureTokenSource(scope, clientID) constructor with default scope
  • GetToken(ctx) method for dynamic token acquisition
• Azure provider managed_identity auth type for containerized deployments
  • System-assigned identity (default) and user-assigned identity via client_id option
  • Configurable token scope via resource option (defaults to https://cognitiveservices.azure.com/.default)
• MockProvider.WithSetHeadersError option for testing authentication failures

Changed:

• Azure provider NewAzure constructor validates auth_type and returns error for unsupported values
• Client layer (execute, executeStream) now propagates SetHeaders errors