Add ClassLoom e2e tests; fix TableView not syncing node attribute changes

[matthiask]

Changes Overview

Two related pieces of work:

1. ClassLoom e2e tests — a new test model and test file covering the ClassLoom extension end-to-end, including paragraph colors (non-combinable) and table layout classes (combinable).
2. TableView attribute sync fix — TableView.update() only re-rendered column widths; node attributes set via addGlobalAttributes (e.g. a class applied by ClassLoom) were visible in the serialized HTML but not in the live editor DOM.

Implementation Approach

Tests:

• ClassLoomProseEditorModel added to the test app with two ClassLoom groups: paragraphColors (non-combinable, targeting paragraph) and tableLayout (combinable, targeting table). Menu groups use button groups for easy Playwright interaction.
• tests/testapp/test_classloom_e2e.py — seven tests asserting toolbar active state, live DOM class presence, and saved model HTML for both non-combinable (apply, switch, remove) and combinable (apply multiple, toggle off one, clear all) behaviours.
• tests/testapp/e2e_utils.py — shared login() helper extracted from test_prose_editor_e2e.py.

TableView fix:
TableView.update() is patched in our Table extension override to call getRenderedAttributes + mergeAttributes on each update, then sync all non-style attributes to tableView.table. The style attribute is left to updateColumns() which manages width/minWidth. An upstream fix has been proposed at ueberdosis/tiptap#7925.

Testing Done

All 47 tests pass (tox -e py313-dj52), including the 7 new e2e tests which assert both the live DOM state and the saved model content.

Verification Steps

1. tox -e py313-dj52 — all tests pass
2. Open /admin/testapp/classloomproseeditormodel/add/, type a paragraph, click a color button — the <p> element should get the class in the live editor DOM and the saved HTML.
3. Insert a table, click a table layout button — the <table> element should get the class in the live DOM and the saved HTML.

Additional Notes

The Upgrades commit (dependency update) is separate and should be pushed to main directly after rebasing on the current tip.