[macOS] UI tests: Avoid test failure if system dialog is not on screen (#2736)

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Task/Issue URL:
https://app.asana.com/1/137249556945/project/1201037661562251/task/1212253220675595?focus=true

### Description

This improves the robustness of the "Set as Default/Add to Dock"
confirmation UI test. At the end of the test we dismiss the system
dialog to prevent it from interfering with other UI tests. However, in
some runs on macOS 13 we observed that it didn't appear (apparently
because we detected that the app was already set as default there, so
the test didn't attempt to set it as default).

We removed macOS 13 from our UI test runs, and confirmed with manual
tests that it behaves as expected on macOS 13. However, for the sake of
avoiding spurious test failures the test now only tries to dismiss the
system dialog if it appears, so it won't fail if the dialog doesn't
appear (since we're not trying to test system behavior).

### Testing Steps

Confirm [UI tests in this workflow
run](https://github.com/duckduckgo/apple-browsers/actions/runs/19858222998)
pass.

### Impact and Risks
None: Internal tooling, documentation

---
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Done](https://app.asana.com/0/1202500774821704/1207634633537039/f) |
[Engineering
Expectations](https://app.asana.com/0/59792373528535/199064865822552) |
[Tech Design
Template](https://app.asana.com/0/59792373528535/184709971311943)

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> Make dismissal of the system default browser dialog in the UI test
conditional on its existence.
> 
> - **UI Tests**
(`macOS/UITests/DefaultBrowserAndDockPromptsUITests.swift`):
> - After confirming the inactive user prompt, check for the system
default browser dialog (`com.apple.coreservices.uiagent`) with
`waitForExistence(...)` and click the second button only if present.
>   - Update the accompanying comment to indicate conditional dismissal.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
ca14098. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Author: rachelmcr

macOS/UITests/DefaultBrowserAndDockPromptsUITests.swift

@@ -53,10 +53,12 @@ final class DefaultBrowserAndDockPromptsUITests: UITestCase {
         app.confirmButton.click()
         XCTAssertTrue(app.inactiveUserPrompt.waitForNonExistence(timeout: UITests.Timeouts.elementExistence), "Inactive user prompt should be dismissed after clicking confirm button")
 
-        // Dismiss the default browser dialog to prevent interference with other tests
+        // Dismiss the default browser dialog, if needed, to prevent interference with other tests
         let coreServicesUIAgent = XCUIApplication(bundleIdentifier: "com.apple.coreservices.uiagent")
         let defaultBrowserDialog = coreServicesUIAgent.dialogs.firstMatch
-        defaultBrowserDialog.buttons.element(boundBy: 1).clickAfterExistenceTestSucceeds()
+        if defaultBrowserDialog.waitForExistence(timeout: UITests.Timeouts.elementExistence) {
+            defaultBrowserDialog.buttons.element(boundBy: 1).click()
+        }
     }
 
     // Note that this test only covers the behavior in the app under test, not the system behavior.