feat(preview): log spec validation failures with raw output

422s from /preview hid the actual reason: zod_message tells which field
was wrong and a 400-char preview of the model output reveals refusals
or non-JSON returns. Both stay in the api log only — never surfaced
to the client unchanged.

packages/llm/src/index.ts

@@ -268,7 +268,13 @@ async function generateWithAnthropic(
     .join('');
   const json = extractJson(text);
   const parsed = GeneratorSpec.safeParse(json);
-  if (!parsed.success) throw new SpecValidationError(parsed.error.message);
+  if (!parsed.success) {
+    // Include a truncated raw preview so the caller (api log) can see whether
+    // the model returned non-JSON / a refusal / a near-miss schema, instead
+    // of just the opaque zod error.
+    const preview = text.slice(0, 400).replace(/\s+/g, ' ');
+    throw new SpecValidationError(`${parsed.error.message} :: raw="${preview}"`);
+  }
   scanForInjection(parsed.data);
   return { spec: parsed.data, source: 'claude' };
 }