fix(cerebras,mistral): strip unsupported client_metadata from downstream requests

Cerebras returns 400 (wrong_api_format) and Mistral returns 422
(extra_forbidden) when the forwarded body contains client_metadata.
Remove it for these two providers before sending downstream.

Co-authored-by: Cursor <cursoragent@cursor.com>

open-sse/executors/default.js

@@ -13,6 +13,13 @@ export class DefaultExecutor extends BaseExecutor {
 
   transformRequest(model, body) {
     const transformed = this.applyJsonSchemaFallback(body);
+
+    if (transformed && typeof transformed === "object") {
+      if (this.provider === "cerebras" || this.provider === "mistral") {
+        delete transformed.client_metadata;
+      }
+    }
+
     return injectReasoningContent({ provider: this.provider, model, body: transformed });
   }