fix: coerce non-string da-metadata values to avoid "[object Object]"

blocks/edit/utils/helpers.js

@@ -329,9 +329,18 @@ export function setDaMetadata(key, value) {
   if (!daMdMap) return;
   if (value === null || value === undefined) {
     daMdMap.delete(key);
-  } else {
-    daMdMap.set(key, value);
+    return;
+  }
+  // The Y.Map serializes to a <div class="da-metadata"> block where values are
+  // interpolated as text. Non-strings (objects, Y types) would render as
+  // "[object Object]" in the saved HTML, so coerce at the boundary.
+  let stringValue = value;
+  if (typeof stringValue !== 'string') {
+    // eslint-disable-next-line no-console
+    console.warn(`setDaMetadata: non-string value for "${key}"`, value);
+    stringValue = typeof value === 'object' ? JSON.stringify(value) : String(value);
   }
+  daMdMap.set(key, stringValue);
 }
 
 export function getDiffLabels() {

test/unit/blocks/edit/utils/helpers.test.js

@@ -632,6 +632,31 @@ describe('daMetadata functions', () => {
       expect(getDaMetadata('key2')).to.be.null;
       expect(getDaMetadata('key3')).to.equal('value3');
     });
+
+    it('Coerces object values to JSON strings to avoid "[object Object]"', () => {
+      const originalWarn = console.warn;
+      console.warn = () => {};
+      try {
+        setDaMetadata('obj-key', { a: 1, b: 'two' });
+        expect(getDaMetadata('obj-key')).to.equal('{"a":1,"b":"two"}');
+      } finally {
+        console.warn = originalWarn;
+      }
+    });
+
+    it('Coerces non-string primitive values to strings', () => {
+      const originalWarn = console.warn;
+      console.warn = () => {};
+      try {
+        setDaMetadata('num-key', 42);
+        expect(getDaMetadata('num-key')).to.equal('42');
+
+        setDaMetadata('bool-key', true);
+        expect(getDaMetadata('bool-key')).to.equal('true');
+      } finally {
+        console.warn = originalWarn;
+      }
+    });
   });
 });