ci: run ep_ai_core and ep_ai_mcp in the with-plugins backend jobs

ep_ai_mcp 1.0.2 broke on core's global express.json() (every /mcp POST
answered -32700) and nothing in core CI noticed. Install both plugins
in the Linux and Windows with-plugins backend jobs so their specs run
against every core change.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01V4cYEvi9Y4BTPntnkV9tzh

Author: JohnMcLear

.github/workflows/backend-tests.yml

@@ -120,6 +120,8 @@ jobs:
         name: Install Etherpad plugins
         run: >
           pnpm add -w
+          ep_ai_core
+          ep_ai_mcp
           ep_align
           ep_author_hover
           ep_cursortrace
@@ -259,6 +261,8 @@ jobs:
         name: Install Etherpad plugins
         run: >
           pnpm add -w
+          ep_ai_core
+          ep_ai_mcp
           ep_align
           ep_author_hover
           ep_cursortrace