feat: Add Mistral models to defineMaxTokens and BrainConfig (#2363)

# Description

Please include a summary of the changes and the related issue. Please
also include relevant motivation and context.

## Checklist before requesting a review

Please delete options that are not relevant.

- [ ] My code follows the style guidelines of this project
- [ ] I have performed a self-review of my code
- [ ] I have commented hard-to-understand areas
- [ ] I have ideally added tests that prove my fix is effective or that
my feature works
- [ ] New and existing unit tests pass locally with my changes
- [ ] Any dependent changes have been merged

## Screenshots (if appropriate):

frontend/lib/helpers/defineMaxTokens.ts

@@ -15,6 +15,12 @@ export const defineMaxTokens = (
       return 4000;
     case "gpt-4-0125-preview":
       return 4000;
+    case "mistral/mistral-small":
+      return 1000;
+    case "mistral/mistral-medium":
+      return 2000;
+    case "mistral/mistral-large":
+      return 2000;
     default:
       return 1000;
   }

frontend/lib/types/BrainConfig.ts

@@ -43,6 +43,9 @@ export const openAiFreeModels = [
   "gpt-3.5-turbo-16k",
   "gpt-4-0125-preview",
   "gpt-3.5-turbo-0125",
+  "mistral/mistral-small",
+  "mistral/mistral-medium",
+  "mistral/mistral-large",
 ] as const;
 
 export const openAiPaidModels = [...openAiFreeModels, "gpt-4"] as const;