fix: Delete the pending Canvas auto-save and create a new revision when resetting a layout

Problem/Motivation

Both reset paths (CanvasResetConfirmForm::submitForm() and the HTMX endpoint CanvasResetController::reset()) empty field_canvas_layout and save, but:

• They never call Canvas's AutoSaveManager::delete(). A pending Canvas auto-save is a full snapshot of the node including the old layout — it stays listed in /canvas/api/v0/auto-saves/pending, and publishing it later silently restores the layout that was just reset.
• They never call setNewRevision(), so on revisionable content the reset overwrites the current default revision in place: no revision trail, no way to see what was removed, even though every normal node save creates a revision.

Steps to reproduce

• Enable canvas_override on a type, edit a node's layout in the Canvas editor (creating an auto-save) but do not publish.
• Reset the layout from the Reset Canvas layout tab.
• Open the Canvas review panel and publish the still-pending change: the "reset" layout comes back.

Proposed resolution

After saving the emptied field, call AutoSaveManager::delete($node) in both reset paths, and save the reset as a new revision with a revision log message, revision user and timestamp.

Remaining tasks

• ✅ File an issue about this project
• ✅ Addition/Change/Update/Fix to this project
• ✅ Testing to ensure no regression
• ✅ Automated unit/functional testing coverage
• ➖ Developer Documentation support on feature change/addition
• ➖ User Guide Documentation support on feature change/addition
• ➖ UX/UI designer responsibilities
• ➖ Accessibility and Readability
• ✅ Reviewed by a human
• ✅ Code review by maintainers
• ✅ Full testing and approval
• ✅ Credit contributors
• ✅ Review with the product owner
• ✅ Update Release Notes
• ✅ Release canvas_override-1.0.0

User interface changes

• N/A

API changes

• N/A

Data model changes

• N/A

Release notes snippet

• Resetting a per-content layout now removes any pending Canvas auto-save (so it cannot be re-published) and records the reset as a new node revision.