Enforce errors on overriding built-in model providers (#12024)

We receive bug reports from users who attempt to override one of the
three built-in model providers (openai, ollama, or lmstuio). Currently,
these overrides are silently ignored. This PR makes it an error to
override them.

## Summary
- add validation for `model_providers` so `openai`, `ollama`, and
`lmstudio` keys now produce clear configuration errors instead of being
silently ignored

codex-rs/core/config.schema.json

@@ -2223,7 +2223,7 @@
         "$ref": "#/definitions/ModelProviderInfo"
       },
       "default": {},
-      "description": "User-defined provider entries that extend/override the built-in list.",
+      "description": "User-defined provider entries that extend the built-in list. Built-in IDs cannot be overridden.",
       "type": "object"
     },
     "model_reasoning_effort": {