Add onCompleted callback system for guaranteed console log delivery

- Add onCompleted() callback registration in RuntimeMessageBridge
- Modify wrapperFunction to call completion callbacks before returning
- Update ConsoleRuntimeProvider to immediate send + completion batch pattern
- Extract DOWNLOADABLE_FILE_RUNTIME_DESCRIPTION from ATTACHMENTS_RUNTIME_DESCRIPTION
- Logs sent immediately (fire-and-forget), unsent logs batched at completion
- Ensures all console logs arrive before tool execution completes

packages/web-ui/src/components/sandbox/RuntimeMessageBridge.ts

@@ -27,6 +27,7 @@ export class RuntimeMessageBridge {
 	private static generateSandboxBridge(sandboxId: string): string {
 		// Returns stringified function that uses window.parent.postMessage
 		return `
+window.__completionCallbacks = [];
 window.sendRuntimeMessage = async (message) => {
     const messageId = 'msg_' + Date.now() + '_' + Math.random().toString(36).substring(2, 9);
 
@@ -57,18 +58,25 @@ window.sendRuntimeMessage = async (message) => {
         }, 30000);
     });
 };
+window.onCompleted = (callback) => {
+    window.__completionCallbacks.push(callback);
+};
 `.trim();
 	}
 
 	private static generateUserScriptBridge(sandboxId: string): string {
 		// Returns stringified function that uses chrome.runtime.sendMessage
 		return `
+window.__completionCallbacks = [];
 window.sendRuntimeMessage = async (message) => {
     return await chrome.runtime.sendMessage({
         ...message,
         sandboxId: ${JSON.stringify(sandboxId)}
     });
 };
+window.onCompleted = (callback) => {
+    window.__completionCallbacks.push(callback);
+};
 `.trim();
 	}
 }