Refactor SSO authentication to support multiple providers and enhance error handling

auth/providers/__init__.py

@@ -0,0 +1,16 @@
+from auth.providers.base import SSOProvider
+from auth.providers.kit import KITProvider
+
+# Registry of available SSO providers
+PROVIDERS: dict[str, type[SSOProvider]] = {
+    "kit": KITProvider,
+}
+
+
+def get_provider(name: str) -> type[SSOProvider]:
+    """Get an SSO provider class by name."""
+    provider = PROVIDERS.get(name.lower())
+    if not provider:
+        available = ", ".join(PROVIDERS.keys())
+        raise ValueError(f"Unknown SSO provider: {name}. Available: {available}")
+    return provider