feat: centralized environment handling, ensures ZEN_MCP_FORCE_ENV_OVERRIDE is honored correctly

fix: updated tests to override env variables they need instead of relying on the current values from .env

providers/custom.py

@@ -1,9 +1,10 @@
 """Custom API provider implementation."""
 
 import logging
-import os
 from typing import Optional
 
+from utils.env import get_env
+
 from .openai_compatible import OpenAICompatibleProvider
 from .openrouter_registry import OpenRouterModelRegistry
 from .shared import ModelCapabilities, ProviderType
@@ -56,9 +57,9 @@ class CustomProvider(OpenAICompatibleProvider):
         """
         # Fall back to environment variables only if not provided
         if not base_url:
-            base_url = os.getenv("CUSTOM_API_URL", "")
+            base_url = get_env("CUSTOM_API_URL", "") or ""
         if not api_key:
-            api_key = os.getenv("CUSTOM_API_KEY", "")
+            api_key = get_env("CUSTOM_API_KEY", "") or ""
 
         if not base_url:
             raise ValueError(