test: add @feature:authorship-bg-color tag for plugins that re-render authorship (#7657)

Plugins like ep_author_neat2 swap Etherpad's coloured-background
authorship indicator for an underline. Their README is explicit
about this; their main is red on the disables-aware test runner
because change_user_color.spec.ts:59 hard-asserts the chat <p>'s
background-color matches the user's colour, which a non-background
rendering legitimately won't satisfy.

Add a second tag (@feature:authorship-bg-color) alongside the
existing @feature:chat so plugins that swap rendering can declare
"disables": ["@feature:authorship-bg-color"] and have this single
spec excluded from pass-1 regression while still running pass-2
honesty (the bg-color assertion fails under the plugin → contract
honoured).

Multi-tag: ep_disable_chat keeps excluding it via @feature:chat;
ep_author_neat2 excludes it via @feature:authorship-bg-color.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: JohnMcLear

src/tests/frontend-new/specs/change_user_color.spec.ts

@@ -57,7 +57,13 @@ test.describe('change user color', function () {
     });
 
   test('Own user color is shown when you enter a chat', {
-    tag: '@feature:chat',
+    // Asserts the user's colour appears as the chat <p> background. Plugins
+    // that re-render authorship as something other than a background (e.g.
+    // ep_author_neat2 swaps the colour-block for an underline) legitimately
+    // make this assertion stop holding, so they declare
+    // `@feature:authorship-bg-color` in their ep.json `disables` list and
+    // the test is excluded from their pass-1 regression run.
+    tag: ['@feature:chat', '@feature:authorship-bg-color'],
   }, async function ({page}) {
 
     const colorOption = page.locator('#options-colorscheck');