Block/Tabs: Fix editor dirty state on reload by removing unnecessary mount-time attribute init

[Mustafabharmal]

What?

Removes the useEffect in tabs/edit.js that initialised editorActiveTabIndex via setAttributes on mount.

Closes #78337.

Why?

editorActiveTabIndex has "role": "local" in block.json — it is never serialised to post content. This means it is always undefined when a saved post is reloaded. The useEffect on mount called setAttributes({ editorActiveTabIndex: activeTabIndex }) without __unstableMarkNextChangeAsNotPersistent, recording a persistent edit every time the editor loaded a post containing a Tabs block. This caused the "Save draft" button to appear instead of the "Saved" checkmark with no user changes.

How?

All consumers of editorActiveTabIndex already gracefully handle undefined by falling back to activeTabIndex via ??:

• tab/edit.js: editorActiveTabIndex ?? activeTabIndex
• tab-panel/edit.js: editorActiveTabIndex ?? activeTabIndex
• remove-tab-toolbar-control.js: tabsAttributes?.editorActiveTabIndex ?? tabsAttributes?.activeTabIndex ?? 0

Removing the useEffect means editorActiveTabIndex stays undefined until the user actively clicks a tab, at which point the child blocks already set it correctly via __unstableMarkNextChangeAsNotPersistent.

Testing Instructions

1. Create a new post.
2. Insert a Tabs block.
3. Save or publish the post.
4. Reload the editor page.
5. Without making any changes, verify the header shows "Saved" with the checkmark — not "Save draft".
6. Click between tabs and verify the active tab still switches correctly.
7. Add/remove tabs and verify they still work correctly.

Screenshots or screencast

Before:

Tab.Block.Testing.mov

After:

Tab.Block.Testing.After.Fix.mov

[github-actions]

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Co-authored-by: Mustafabharmal <mustafabharmal@git.wordpress.org>
Co-authored-by: t-hamano <wildworks@git.wordpress.org>

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[t-hamano]

LGTM! This effect is redundant and should be removable without changing the editor's behavior.

As a side note, it would be great if editorActiveTabIndex could be completely removed. Attributes are not intended to indicate the editor's state.