fix: change mcp_api_keys to string field to avoid JSON parsing

Pydantic was trying to parse List[str] as JSON from env vars.
Changed to use a string field (mcp_api_keys_raw) and parse manually
in model_validator, then expose as property.

This fixes the JSONDecodeError when reading MCP_API_KEYS from .env

src/aegis_gitea_mcp/config.py

@@ -15,6 +15,8 @@ class Settings(BaseSettings):
         env_file_encoding="utf-8",
         case_sensitive=False,
         extra="ignore",
+        # Don't try to parse env vars as JSON for complex types
+        env_parse_none_str="null",
     )
 
     # Gitea configuration
@@ -72,9 +74,10 @@ class Settings(BaseSettings):
         default=True,
         description="Enable API key authentication (disable only for testing)",
     )
-    mcp_api_keys: List[str] = Field(
-        default_factory=list,
-        description="List of valid API keys for MCP access (comma-separated in env)",
+    mcp_api_keys_raw: str = Field(
+        default="",
+        description="Comma-separated API keys for MCP access",
+        alias="MCP_API_KEYS",
     )
     max_auth_failures: int = Field(
         default=5,
@@ -105,34 +108,26 @@ class Settings(BaseSettings):
             raise ValueError("gitea_token cannot be empty or whitespace")
         return v.strip()
 
-    @field_validator("mcp_api_keys", mode="before")
-    @classmethod
-    def parse_api_keys(cls, v: object) -> List[str]:
-        """Parse API keys from comma-separated string or list."""
-        if v is None:
-            return []
-        if isinstance(v, str):
-            # Empty string returns empty list
-            if not v.strip():
-                return []
-            # Split by comma and strip whitespace
-            keys = [key.strip() for key in v.split(",") if key.strip()]
-            return keys
-        elif isinstance(v, list):
-            return v
-        return []
-
     @model_validator(mode="after")
-    def validate_api_keys_if_auth_enabled(self) -> "Settings":
-        """Validate API keys if authentication is enabled."""
-        if self.auth_enabled and not self.mcp_api_keys:
+    def validate_and_parse_api_keys(self) -> "Settings":
+        """Parse and validate API keys if authentication is enabled."""
+        # Parse comma-separated keys into list
+        keys = []
+        if self.mcp_api_keys_raw and self.mcp_api_keys_raw.strip():
+            keys = [key.strip() for key in self.mcp_api_keys_raw.split(",") if key.strip()]
+        
+        # Store in a property we'll access
+        object.__setattr__(self, "_mcp_api_keys", keys)
+        
+        # Validate if auth is enabled
+        if self.auth_enabled and not keys:
             raise ValueError(
                 "At least one API key must be configured when auth_enabled=True. "
                 "Set MCP_API_KEYS environment variable or disable auth with AUTH_ENABLED=false"
             )
         
         # Validate key format (at least 32 characters for security)
-        for key in self.mcp_api_keys:
+        for key in keys:
             if len(key) < 32:
                 raise ValueError(
                     f"API keys must be at least 32 characters long. "
@@ -140,6 +135,11 @@ class Settings(BaseSettings):
                 )
         
         return self
+    
+    @property
+    def mcp_api_keys(self) -> List[str]:
+        """Get parsed list of API keys."""
+        return getattr(self, "_mcp_api_keys", [])
 
     @property
     def gitea_base_url(self) -> str: