fix(cloudflare-ai): flatten content-part arrays to string to avoid oneOf 400 (#1926)

Workers AI rejects OpenAI content-part array shape; flatten text parts
to a plain string per message before sending.

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

open-sse/translator/concerns/paramSupport.js

@@ -10,6 +10,8 @@ const STRIP_RULES = [
   { provider: "github", match: /gpt-5\.4/i, drop: ["temperature"] },
   // GitHub Copilot Claude (except opus/sonnet 4.6): thinking + reasoning_effort rejected. #713
   { provider: "github", match: (m) => /claude/i.test(m) && !/claude.*(opus|sonnet).*4\.6/i.test(m), drop: ["thinking", "reasoning_effort"] },
+  // Cloudflare Workers AI: content must be plain string, rejects OpenAI content-part array (#1926)
+  { provider: "cloudflare-ai", flattenContent: true },
 ];
 
 // Test a rule's match (regex or predicate) against the model id.
@@ -26,6 +28,16 @@ export function stripUnsupportedParams(provider, model, body) {
     for (const key of rule.drop) {
       if (body[key] !== undefined) delete body[key];
     }
+    // CF Workers AI oneOf root schema only accepts content as plain string (#1926)
+    if (rule.flattenContent && Array.isArray(body.messages)) {
+      for (const msg of body.messages) {
+        if (msg && Array.isArray(msg.content)) {
+          msg.content = msg.content
+            .map(b => (b?.type === "text" && typeof b.text === "string") ? b.text : "")
+            .join("");
+        }
+      }
+    }
   }
   return body;
 }