Address PR #1327 review: replace last().click() with getByRole

Harden the remaining Cancel-button selector in the edit-modal cancel
test — `.oc-modal button:has-text('Cancel')` + `.last()` silently picks
whichever Cancel is last in DOM order, which is fragile if a future
render adds a second cancel-like control. Switch to
`getByRole("button", { name: "Cancel", exact: true })` to match the
Save-Changes pattern used earlier in the file.

Author: JSv4

frontend/tests/CorpusAgentManagement.ct.tsx

@@ -744,7 +744,7 @@ test.describe("CorpusAgentManagement", () => {
     ).toBeVisible({ timeout: 5000 });
 
     // Click Cancel in the modal footer
-    await page.locator(".oc-modal button:has-text('Cancel')").last().click();
+    await page.getByRole("button", { name: "Cancel", exact: true }).click();
 
     await expect(
       page.getByText(`Edit Agent Configuration: ${sampleAgent.name}`, {