Fix: Edit-post back button tooltip position

[DarkMatter-999]

What?

Part of #73785

This PR updates the tooltipPosition of the Fullscreen Mode Close button (the "Back" button in the top left of the editor) from middle right to bottom.

Why?

The previous middle right positioning caused the tooltip to overlap with the block inserter button. Changing the position to bottom ensures the tooltip appears directly beneath the button, which is standard for header-level icons and prevents visual clashing with the other elements.

How?

Updated the tooltipPosition prop for FullscreenModeClose component to use the value bottom.

Testing Instructions

1. Open the Block editor.
2. Enable Fullscreen Mode in the editor (Options > View > Fullscreen Mode).
3. Hover over the WordPress (or Site) icon in the top left corner.
4. Verify that the "View Posts" (or similar) tooltip now appears below the button rather than to the right of it.

Screenshots or screencast

Before:

edit-post-back-button-tooltip.mov

After:

edit-post-back-button-tooltip-fix.mov

Use of AI Tools

I used Gemini 3 Flash to help format this PR description and summarize the logic implementation based on my git diff.

cc: @ciampo, @mirka

[ciampo]

@DarkMatter-999 can you just check that the tooltip still looks good when the string of text gets longer?

[DarkMatter-999]

This is how it looks, does not overflow to the edges.

[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

If you're merging code through a pull request on GitHub, copy and paste the following into the bottom of the merge commit message.

Co-authored-by: DarkMatter-999 <lakshyajeet@git.wordpress.org>
Co-authored-by: ciampo <mciampini@git.wordpress.org>

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.

[ciampo]

LGTM 🚀