feat(provider): add CodeBuddy CN provider (copilot.tencent.com)

Add Tencent CodeBuddy CN (codebuddy-cn) OAuth provider with full support:
OAuth login (GET poll with state query param), token refresh, 15-model
catalog, /v2 inference endpoint, forced streaming, OpenAI-style reasoning,
and per-model capabilities. Renamed from codebuddy to codebuddy-cn to allow
a future codebuddy-ai variant.

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

src/app/api/providers/[id]/test/testUtils.js

@@ -90,7 +90,7 @@ const OAUTH_TEST_CONFIG = {
     authHeader: "Authorization",
     authPrefix: "Bearer ",
   },
-  codebuddy: { tokenExists: true },
+  "codebuddy-cn": { tokenExists: true },
 };
 
 async function probeClineAccessToken(accessToken) {