fix: report unknown page id before last-page guard in close_page (#3042)

Fixes #2991

On a session with a single open page, calling `close_page` with a pageId
that doesn't exist returned "The last open page cannot be closed". That
points at the wrong problem.

`McpContext.closePage` now looks the page up first and only then applies
the last-page guard. An unknown id fails with "No page found". Closing
the real last page behaves as before.

Added a test to tests/McpContext.test.ts. It fails on main and passes
with this change.

src/McpContext.ts

@@ -346,10 +346,10 @@ export class McpContext implements Context {
     return mcpPage;
   }
   async closePage(pageId: number): Promise<void> {
+    const page = this.getPageById(pageId);
     if (this.#mcpPages.size === 1) {
       throw new Error(CLOSE_PAGE_ERROR);
     }
-    const page = this.getPageById(pageId);
     this.#mcpPages.delete(page.target);
     await page.close();
   }

tests/McpContext.test.ts

@@ -384,6 +384,18 @@ describe('McpContext', () => {
     });
   });
 
+  it('reports an unknown page id before the last-page guard on close', async () => {
+    await withMcpContext(async (_response, context) => {
+      const page = context.getSelectedMcpPage();
+      await assert.rejects(context.closePage(99), /No page found/);
+      await assert.rejects(
+        context.closePage(page.id),
+        /The last open page cannot be closed/,
+      );
+      assert.ok(!page.pptrPage.isClosed());
+    });
+  });
+
   it('reports the reconnect notice once', async () => {
     await withBrowser(async browser => {
       const context = await McpContext.from(