feat(github): resolve Copilot model catalog from upstream

Fetch the live model list from the Copilot /models endpoint and surface
it through /v1/models, replacing the hardcoded github entry so newly
shipped models appear without a code change. Catalog is cached per
credential and the Copilot token is refreshed on 401/403 before retry.

Also raise the connectivity-test budget to max_tokens:16, since Claude on
Copilot emits no choices at max_tokens:1 and produced a false negative.

Co-authored-by: Cursor <cursoragent@cursor.com>

src/app/api/models/test/ping.js

@@ -135,7 +135,9 @@ export async function pingModelByKind(model, kind, baseUrl = `http://127.0.0.1:$
     headers,
     body: JSON.stringify({
       model,
-      max_tokens: 1,
+      // Claude-on-Copilot returns empty choices at max_tokens:1 (budget is spent
+      // before a content token emits), so a 1-token probe yields a false negative.
+      max_tokens: 16,
       stream: false,
       messages: [{ role: "user", content: "hi" }],
     }),