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

config.py

@@ -8,7 +8,7 @@ constants used throughout the application.
 Configuration values can be overridden by environment variables where appropriate.
 """
 
-import os
+from utils.env import get_env
 
 # Version and metadata
 # These values are used in server responses and for tracking releases
@@ -25,7 +25,7 @@ __author__ = "Fahad Gilani"
 # This should be a stable, high-performance model suitable for code analysis
 # Can be overridden by setting DEFAULT_MODEL environment variable
 # Special value "auto" means Claude should pick the best model for each task
-DEFAULT_MODEL = os.getenv("DEFAULT_MODEL", "auto")
+DEFAULT_MODEL = get_env("DEFAULT_MODEL", "auto") or "auto"
 
 # Auto mode detection - when DEFAULT_MODEL is "auto", Claude picks the model
 IS_AUTO_MODE = DEFAULT_MODEL.lower() == "auto"
@@ -61,7 +61,7 @@ TEMPERATURE_CREATIVE = 0.7  # For architecture, deep thinking
 # Thinking Mode Defaults
 # DEFAULT_THINKING_MODE_THINKDEEP: Default thinking depth for extended reasoning tool
 # Higher modes use more computational budget but provide deeper analysis
-DEFAULT_THINKING_MODE_THINKDEEP = os.getenv("DEFAULT_THINKING_MODE_THINKDEEP", "high")
+DEFAULT_THINKING_MODE_THINKDEEP = get_env("DEFAULT_THINKING_MODE_THINKDEEP", "high") or "high"
 
 # Consensus Tool Defaults
 # Consensus timeout and rate limiting settings
@@ -117,7 +117,7 @@ def _calculate_mcp_prompt_limit() -> int:
         Maximum character count for user input prompts
     """
     # Check for Claude's MAX_MCP_OUTPUT_TOKENS environment variable
-    max_tokens_str = os.getenv("MAX_MCP_OUTPUT_TOKENS")
+    max_tokens_str = get_env("MAX_MCP_OUTPUT_TOKENS")
 
     if max_tokens_str:
         try:
@@ -143,7 +143,7 @@ MCP_PROMPT_SIZE_LIMIT = _calculate_mcp_prompt_limit()
 # Examples: "fr-FR", "en-US", "zh-CN", "zh-TW", "ja-JP", "ko-KR", "es-ES",
 # "de-DE", "it-IT", "pt-PT"
 # Leave empty for default language (English)
-LOCALE = os.getenv("LOCALE", "")
+LOCALE = get_env("LOCALE", "") or ""
 
 # Threading configuration
 # Simple in-memory conversation threading for stateless MCP environment