Optimize media manager polling and product info retrieval

- Remove recursive polling in MediaManager.html; context is now loaded once at startup.
- Optimize getSelectedProductInfo in mediaHandlers.ts to reduce SpreadsheetApp API calls.
- Update related tests to match new optimization.

src/MediaManager.html

@@ -1206,8 +1206,8 @@
      */
     var controller = {
       init() {
-        // Start polling for SKU selection
-        setInterval(() => this.checkSku(), 2000);
+        // Initialize by checking SKU once
+        // Since this is a modal, the selection cannot change during the session.
         this.checkSku();
       },
 
@@ -1222,10 +1222,19 @@
               this.loadMedia();
             } else if (!sku && !state.sku) {
               if (document.getElementById('error-ui').style.display !== 'flex') {
-                this.loadMedia();
+                this.loadMedia(); // Likely to trigger error UI
               }
             }
           })
+          .withFailureHandler(e => {
+            console.warn("SKU check failed", e);
+            // If it fails once at startup, we probably should alert or retry once,
+            // but for now let's just leave it. If it fails, the UI might hang on "Loading..."
+            // potentially better to trigger error UI?
+            if (document.getElementById('loading-ui').style.display !== 'none') {
+              alert("Failed to load product info: " + e.message);
+            }
+          })
           .getSelectedProductInfo();
       },