Navigation Block: Polish Block Hooks integration

[ockham]

What?

WIP. Potential follow-up to #59021.

Why?

For more consistency in some of our helpers.

How?

By stripping the mock Navigation root block from the return value of block_core_navigation_insert_hooked_blocks.

The downside is that it's now less consistent with block_core_navigation_set_ignored_hooked_blocks_metadata. We might want to figure out a way around that; possibly by inlining the latter into the function that calls it.

Testing Instructions

See e.g. #59021.

[ockham]

cc/ @tjcafferkey -- Curious if you think this makes sense or if we should abandon it 🙂

[github-actions]

Flaky tests detected in 5207cac.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/7960966803
📝 Reported issues:

• [Flaky Test] default to a list of pages if there are no menus #47732 in /test/e2e/specs/editor/blocks/navigation.spec.js