refactor(core): 🔨 use static class attribute for provider skipping flag

Convert `DynamicOpenAICompatibleProvider.skip_cost_calculation` from an instance method to a static class attribute.

This allows the `UsageManager` to check the property directly against the provider class definition via `getattr`, avoiding unnecessary method overhead and potential instantiation checks for a simple flag.

src/rotator_library/providers/__init__.py

@@ -16,6 +16,9 @@ class DynamicOpenAICompatibleProvider:
     Created at runtime for providers with API_BASE environment variables.
     """
 
+    # Class attribute - no need to instantiate
+    skip_cost_calculation: bool = True
+
     def __init__(self, provider_name: str):
         self.provider_name = provider_name
         # Get API base URL from environment
@@ -30,10 +33,6 @@ class DynamicOpenAICompatibleProvider:
 
         self.model_definitions = ModelDefinitions()
 
-    def skip_cost_calculation(self) -> bool:
-        """Custom providers should skip cost calculation."""
-        return True
-
     def get_models(self, api_key: str, client):
         """Delegate to OpenAI-compatible provider implementation."""
         from .openai_compatible_provider import OpenAICompatibleProvider

src/rotator_library/usage_manager.py

@@ -344,7 +344,10 @@ class UsageManager:
                     provider_name = model.split("/")[0]
                     provider_plugin = PROVIDER_PLUGINS.get(provider_name)
 
-                    if provider_plugin and provider_plugin.skip_cost_calculation():
+                    # Check class attribute directly - no need to instantiate
+                    if provider_plugin and getattr(
+                        provider_plugin, "skip_cost_calculation", False
+                    ):
                         lib_logger.debug(
                             f"Skipping cost calculation for provider '{provider_name}' (custom provider)."
                         )