test: de-flake ViewHeader connect/disconnect animation assertion (#1699)

The server name and Disconnect button are removed by two independent
Mantine Transition exits. The test waitFor'd the server-name removal, then
did a synchronous queryByRole check for the button — on a slow runner the
button's exit lags the name's, so it was still in the DOM at the check.
Wrap the button "removed" assertion in its own waitFor.

Part 2 of #1699 (TUI OAuth step-up) was already fixed in PR #1742 via
POLL_TRIES=100.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XNdjEPKLG637X8YmhDiEk5

clients/web/src/components/groups/ViewHeader/ViewHeader.test.tsx

@@ -355,12 +355,18 @@ describe("ViewHeader", () => {
           ?.getAttribute("data-anim"),
       ).toBe("out");
 
+      // The server name and the Disconnect button are removed by two
+      // independent Transition exits; on a slow runner the button's exit can
+      // lag the name's, so each "removed" assertion gets its own waitFor rather
+      // than assuming the two transitions complete in lockstep (#1699).
       await waitFor(() =>
         expect(screen.queryByText("my-mcp-server")).not.toBeInTheDocument(),
       );
-      expect(
-        screen.queryByRole("button", { name: "Disconnect from server" }),
-      ).not.toBeInTheDocument();
+      await waitFor(() =>
+        expect(
+          screen.queryByRole("button", { name: "Disconnect from server" }),
+        ).not.toBeInTheDocument(),
+      );
     });
 
     it("renders the dark-scheme icon/logo branch under a dark color scheme", () => {