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>

tests/__baseline__/alias-baseline.json

@@ -105,7 +105,7 @@
     "claude": "cc",
     "cline": "cl",
     "cloudflare-ai": "cloudflare-ai",
-    "codebuddy": "codebuddy",
+    "codebuddy-cn": "codebuddy-cn",
     "codex": "cx",
     "cohere": "cohere",
     "commandcode": "commandcode",
@@ -231,4 +231,4 @@
     "xiaomi-mimo",
     "xiaomi-tokenplan"
   ]
-}
+}