E2E: Add test coverage for Navigation block fallback behavior

[himanshupathak95]

What?

Closes #36825

Adds E2E test coverage for Navigation block fallback behavior, ensuring the navigation block falls back appropriately when no menus exist and uses the first non-empty menu as a fallback.

Why?

In #36740, functionality was added to provide fallback behavior for the Navigation block, determining what users see when there are no menus or when menus become unavailable. This PR adds dedicated test coverage to ensure this new fallback behavior remains stable and works as expected.

How?

Added two new E2E tests in the navigation.spec.js file:

• Tests that the block falls back to a Page List when no menus exist
• Tests that the block uses the first non-empty menu as a fallback

Testing Instructions

• Run

npm run test:e2e -- editor/blocks/navigation.spec.js

• Verify that all tests pass

To manually verify the behavior:

• Create a new post
• Insert a Navigation block with no menus present - should show Page List
• Create a menu with content and insert a Navigation block - should show menu-content

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Co-authored-by: himanshupathak95 <abcd95@git.wordpress.org>
Co-authored-by: getdave <get_dave@git.wordpress.org>

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