test(mcp): retarget the step-number selector at the brand ramp

`numbersIn` located step markers by grepping the rendered markup for
`border-violet-600`. Widgets now paint accents with the tenant brand ramp,
so that class no longer exists and the matcher returned [] — the assertion
failed even though step numbering (the #561 regression this guards) is
unchanged.

Anchor on `border-[var(--brand-600)]` instead, which follows the accent
rather than a fixed hue.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017sLtN12VfnLziCpst3Pfo2

Author: guillermoscript

tests/unit/mcp-lesson-mdx.test.ts

@@ -134,8 +134,11 @@ describe('LessonBody', () => {
       '  <Step title="Three">c</Step>',
       '</Steps>',
     ].join('\n')
+    // The step marker is the only element carrying the brand border colour.
+    // It was `border-violet-600` until widgets moved to the tenant brand ramp
+    // (`--brand-*`), so this selector tracks the accent, not a fixed hue.
     const numbersIn = (markup: string) =>
-      [...markup.matchAll(/border-violet-600[^>]*>(\d+)</g)].map((m) => m[1])
+      [...markup.matchAll(/border-\[var\(--brand-600\)\][^>]*>(\d+)</g)].map((m) => m[1])
 
     // Rendering the same content twice must not drift: a counter incremented
     // during render double-counts under React's development double-invoke and