fix(stt): address review feedback — reserved fields guard, schema validation, docs

- Filter reserved fields (file, model, language) from extraParams before merge
- Change extraParams schema to z.record(z.union([z.string(), z.number(), z.boolean()]))
- Add regex validation for language field in Zod schema
- Add JSDoc note about extraParams in openAIProvider
- Add comment clarifying language/extraParams are OpenAI-only (not Azure)
- Remove unnecessary optional chaining on sttSchema

packages/data-provider/src/config.ts

@@ -478,10 +478,14 @@ const sttOpenaiSchema = z.object({
   url: z.string().optional(),
   apiKey: z.string(),
   model: z.string(),
-  language: z.string().optional(),
-  extraParams: z.record(z.unknown()).optional(),
+  language: z
+    .string()
+    .regex(/^[a-z]{2}(-[a-z]{2})?$/)
+    .optional(),
+  extraParams: z.record(z.union([z.string(), z.number(), z.boolean()])).optional(),
 });
 
+/** Note: language and extraParams are only supported for the OpenAI provider. */
 const sttAzureOpenAISchema = z.object({
   instanceName: z.string(),
   apiKey: z.string(),