refactor(client): 🔨 make credentials optional and centralize validation

The `RotatingClient` now accepts `None` for `api_keys` and `oauth_credentials` in its constructor, improving flexibility and allowing explicit omission of one credential type.

This commit centralizes the credential verification logic within the client's `__init__` method:
- Filters input dictionaries to remove providers associated with empty key lists.
- Implements a single unified check ensuring that at least one valid credential (API keys or OAuth paths) is provided and non-empty.
- Removes the now redundant credential check from `src/proxy_app/main.py`.

src/proxy_app/main.py

@@ -175,9 +175,6 @@ async def lifespan(app: FastAPI):
     cred_manager = CredentialManager(os.environ)
     oauth_credentials = cred_manager.discover_and_prepare()
 
-    if not api_keys and not oauth_credentials:
-        raise ValueError("No provider API keys or OAuth credentials found.")
-
     if not skip_oauth_init and oauth_credentials:
         logging.info("Starting OAuth credential validation and deduplication...")
         processed_emails = {}  # email -> {provider: path}

src/rotator_library/client.py

@@ -40,8 +40,8 @@ class RotatingClient:
     """
     def __init__(
         self,
-        api_keys: Dict[str, List[str]],
-        oauth_credentials: Dict[str, List[str]],
+        api_keys: Optional[Dict[str, List[str]]] = None,
+        oauth_credentials: Optional[Dict[str, List[str]]] = None,
         max_retries: int = 2,
         usage_file_path: str = "key_usage.json",
         configure_logging: bool = True,
@@ -66,8 +66,16 @@ class RotatingClient:
         else:
             lib_logger.propagate = False
 
-        if not api_keys:
-            raise ValueError("API keys dictionary cannot be empty.")
+        api_keys = api_keys or {}
+        oauth_credentials = oauth_credentials or {}
+
+        # Filter out providers with empty lists of credentials to ensure validity
+        api_keys = {provider: keys for provider, keys in api_keys.items() if keys}
+        oauth_credentials = {provider: paths for provider, paths in oauth_credentials.items() if paths}
+
+        if not api_keys and not oauth_credentials:
+            raise ValueError("No valid credentials provided. Either 'api_keys' or 'oauth_credentials' must be provided and non-empty.")
+
         self.api_keys = api_keys
         self.credential_manager = CredentialManager(oauth_credentials)
         self.oauth_credentials = self.credential_manager.discover_and_prepare()