fix: wire Documentation activity-bar button to correct sidebar tab (fixes #21919)

- Changed help:docs event handler in CortexDesktopLayout to open the docs
  sidebar panel (setSidebarTab/setSidebarCollapsed) instead of opening an
  external URL, making the menu-bar Documentation action consistent with
  the activity-bar button.
- Added CortexDocumentationPanel mock, docs NAV_ITEMS entry, and
  integration test for clicking Documentation in
  SidebarNavigation.integration.test.tsx.

Author: echobt

src/components/cortex/__tests__/SidebarNavigation.integration.test.tsx

@@ -122,8 +122,8 @@ const NAV_ITEMS: ActivityBarItem[] = [
   { id: "plugins", icon: "plugins", label: "Plugins" },
   { id: "agents", icon: "users", label: "AI Agents" },
   { id: "extensions", icon: "grid", label: "Extensions" },
-  { id: "docs", icon: "book", label: "Documentation" },
   { id: "themes", icon: "brush", label: "Themes" },
+  { id: "docs", icon: "book", label: "Documentation" },
 ];
 
 function SidebarIntegrationWrapper(props: { initialTab?: SidebarTab; initialCollapsed?: boolean }) {
@@ -304,30 +304,30 @@ describe("SidebarNavigation Integration", () => {
       expect(panel.textContent).toContain("Plugins Panel");
     });
 
-    it("clicking 'docs' shows documentation panel", async () => {
+    it("clicking 'account' shows account panel", async () => {
       const { container, findByTestId } = render(() => <SidebarIntegrationWrapper />);
 
-      const docsButton = container.querySelector('button[aria-label="Documentation"]');
-      expect(docsButton).toBeTruthy();
+      const avatarButton = container.querySelector('button[aria-label="User account"]');
+      expect(avatarButton).toBeTruthy();
 
-      await fireEvent.click(docsButton!);
+      await fireEvent.click(avatarButton!);
 
-      const panel = await findByTestId("panel-docs");
+      const panel = await findByTestId("panel-account");
       expect(panel).toBeTruthy();
-      expect(panel.textContent).toContain("Documentation Panel");
+      expect(panel.textContent).toContain("Account Panel");
     });
 
-    it("clicking 'account' shows account panel", async () => {
+    it("clicking 'docs' shows documentation panel", async () => {
       const { container, findByTestId } = render(() => <SidebarIntegrationWrapper />);
 
-      const avatarButton = container.querySelector('button[aria-label="User account"]');
-      expect(avatarButton).toBeTruthy();
+      const docsButton = container.querySelector('button[aria-label="Documentation"]');
+      expect(docsButton).toBeTruthy();
 
-      await fireEvent.click(avatarButton!);
+      await fireEvent.click(docsButton!);
 
-      const panel = await findByTestId("panel-account");
+      const panel = await findByTestId("panel-docs");
       expect(panel).toBeTruthy();
-      expect(panel.textContent).toContain("Account Panel");
+      expect(panel.textContent).toContain("Documentation Panel");
     });
   });