fix: use model_validator for API keys validation

Changed from field_validator to model_validator to properly access
auth_enabled field during validation. This fixes the SettingsError
when parsing mcp_api_keys from environment variables.

Also improved handling of empty strings and None values.

src/aegis_gitea_mcp/config.py

@@ -3,7 +3,7 @@
 from pathlib import Path
 from typing import List, Optional
 
-from pydantic import Field, HttpUrl, field_validator
+from pydantic import Field, HttpUrl, field_validator, model_validator
 from pydantic_settings import BaseSettings, SettingsConfigDict
 
 
@@ -109,7 +109,12 @@ class Settings(BaseSettings):
     @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
@@ -117,28 +122,24 @@ class Settings(BaseSettings):
             return v
         return []
 
-    @field_validator("mcp_api_keys")
-    @classmethod
-    def validate_api_keys(cls, v: List[str], info) -> List[str]:
+    @model_validator(mode="after")
+    def validate_api_keys_if_auth_enabled(self) -> "Settings":
         """Validate API keys if authentication is enabled."""
-        # Get auth_enabled from values (it's already been processed)
-        auth_enabled = info.data.get("auth_enabled", True)
-        
-        if auth_enabled and not v:
+        if self.auth_enabled and not self.mcp_api_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 v:
+        for key in self.mcp_api_keys:
             if len(key) < 32:
                 raise ValueError(
                     f"API keys must be at least 32 characters long. "
                     f"Use scripts/generate_api_key.py to generate secure keys."
                 )
         
-        return v
+        return self
 
     @property
     def gitea_base_url(self) -> str: