fix: address Copilot review comments (round 4)

- Update AsyncSerializer JSDoc to document rejection behavior: the
  returned promise can reject, rethrowing the first error after all
  pending executions have been processed
- Wrap reloadMkdocsConfig body in try/catch to prevent unhandled
  promise rejections from propagating to the extension host; errors
  are logged to the output channel and shown as a user-friendly message
- Rename coalescing test to 'should coalesce multiple pending calls
  into a single additional execution' for accuracy

Author: jcouball

src/asyncSerializer.ts

@@ -35,7 +35,9 @@ export class AsyncSerializer {
    * @returns A promise that resolves when this call has been acknowledged
    *          (either executed immediately or queued for a coalesced re-run). Awaiting
    *          this promise does not guarantee a distinct execution of {@link fn} for
-   *          this call; coalesced calls share execution cycles.
+   *          this call; coalesced calls share execution cycles. The promise rejects
+   *          if any execution performed during this cycle throws, and rethrows the
+   *          first error encountered after pending executions have been processed.
    */
   async execute(fn: () => Promise<void>): Promise<void> {
     // If already executing, store latest fn for the coalesced re-run and return immediately

src/extension.ts

@@ -50,19 +50,25 @@ export async function activate(context: vscode.ExtensionContext) {
 
   // Reload config and refresh diagnostics when mkdocs config changes
   const reloadMkdocsConfig = async () => {
-    await configReloadSerializer.execute(async () => {
-      const currentFolders = vscode.workspace.workspaceFolders;
-      if (currentFolders && currentFolders.length > 0) {
-        await diagnosticManager.loadMkdocsConfig(currentFolders[0].uri.fsPath);
-
-        // Refresh diagnostics for all open markdown files
-        vscode.window.visibleTextEditors
-          .filter(editor => editor.document.languageId === 'markdown')
-          .forEach(editor => {
-            diagnosticManager.updateDiagnostics(editor.document);
-          });
-      }
-    });
+    try {
+      await configReloadSerializer.execute(async () => {
+        const currentFolders = vscode.workspace.workspaceFolders;
+        if (currentFolders && currentFolders.length > 0) {
+          await diagnosticManager.loadMkdocsConfig(currentFolders[0].uri.fsPath);
+
+          // Refresh diagnostics for all open markdown files
+          vscode.window.visibleTextEditors
+            .filter(editor => editor.document.languageId === 'markdown')
+            .forEach(editor => {
+              diagnosticManager.updateDiagnostics(editor.document);
+            });
+        }
+      });
+    } catch (error) {
+      const errorMessage = error instanceof Error ? error.message : String(error);
+      outputChannel.appendLine(`Failed to reload MkDocs configuration: ${errorMessage}`);
+      void vscode.window.showErrorMessage('Failed to reload MkDocs configuration. See output for details.');
+    }
   };
 
   mkdocsWatcher.onDidCreate(reloadMkdocsConfig);

src/test/unit/asyncSerializer.test.ts

@@ -40,7 +40,7 @@ describe('AsyncSerializer', () => {
     assert.strictEqual(maxInFlight, 1, 'At most one execution should be in flight at a time');
   });
 
-  it('should coalesce multiple pending calls into single execution', async () => {
+  it('should coalesce multiple pending calls into a single additional execution', async () => {
     const serializer = new AsyncSerializer();
     let executionCount = 0;