Media: Hide single-item media menus

[Sukhendu2002]

Trac ticket: https://core.trac.wordpress.org/ticket/60666

This PR fixes an issue where the media modal shown from the Gallery block displays an unnecessary left-side Actions menu containing only Create Gallery.

Because Create Gallery is the only available menu item in this flow and is already the active state, the menu looks interactive but does not provide any useful action.

This updates media menu visibility so that menus with fewer than two visible actionable items are hidden.

The fix avoids the regression from the previous approach by still allowing the media menu mode to render normally before visibility is evaluated. This keeps other media modal flows, such as the Classic block media modal, from losing their menu setup.

Testing

1. Create a new post or page.
2. Add a Gallery block.
3. Select Media Library.
4. Confirm the left Actions / Create Gallery menu is not shown.
5. Confirm images can still be selected and a gallery can still be created.
6. Add a Classic block and use Add Media.
7. Confirm the normal media menu still appears and media insertion still works.

Use of AI Tools

AI assistance: Yes
Tool(s): Codex
Model(s): GPT-5.6
Used for: PR description drafting

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