Tabs: Disable anchor support on Tab Menu Item

[mikachan]

What?

Follow-up to #75954

Disables the "HTML Anchor" field in the Advanced panel for the Tab Menu Item block to prevent users from editing or removing the auto-generated anchor value.

Why?

Editing the anchor on the Tab Menu Item block breaks the sync between the tab panel and its label. The Tab Menu Item block uses the anchor attribute (e.g., tab-1-button) to identify which tab panel it controls. This value is set automatically when the Tabs block is inserted. Because "anchor": true was set in the block's supports, an editable "HTML Anchor" field appeared in the Advanced inspector panel. Editing or clearing this value breaks the connection between the menu item and its corresponding tab panel.

How?

Removed "anchor": true from the block's supports in block.json, which removes the editable "HTML Anchor" UI control. The anchor attribute is now declared explicitly under attributes instead, so the value continues to be stored and used internally to match menu items to tab panels; it just can no longer be edited by the user.

Testing Instructions

1. Create a new post or page
2. Insert a Tabs block
3. Select one of the Tab Menu Item blocks
4. Open the Advanced panel in the block inspector on the right
5. Confirm there is no "HTML Anchor" field present
6. Verify the tabs still function as expected

[github-actions]

Flaky tests detected in 2a41d28.
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/23002850068
📝 Reported issues:

• [Flaky Test] should undo bold #48558 in /test/e2e/specs/editor/various/undo.spec.js

[scruffian]

LGTM

[github-actions]

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Co-authored-by: mikachan <mikachan@git.wordpress.org>
Co-authored-by: scruffian <scruffian@git.wordpress.org>

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