fix(providers): remove Qwen provider support

Qwen OAuth flow (portal.qwen.ai) stopped working reliably; drop the
executor, registry entry, OAuth provider/service, token refresh
profile, usage handler, and related test coverage and baselines.

tests/__baseline__/oauth-urls-baseline.json

@@ -12,10 +12,6 @@
       "token": "https://api.anthropic.com/v1/oauth/token",
       "auth": "https://api.anthropic.com/v1/oauth/authorize"
     },
-    "qwen": {
-      "token": "https://chat.qwen.ai/api/v1/oauth2/token",
-      "auth": "https://chat.qwen.ai/api/v1/oauth2/device/code"
-    },
     "iflow": {
       "token": "https://iflow.cn/oauth/token",
       "auth": "https://iflow.cn/oauth"
@@ -29,7 +25,6 @@
   "tokenUrls": {
     "claude": "https://api.anthropic.com/v1/oauth/token",
     "codex": "https://auth.openai.com/oauth/token",
-    "qwen": "https://chat.qwen.ai/api/v1/oauth2/token",
     "iflow": "https://iflow.cn/oauth/token",
     "kiro": "https://prod.us-east-1.auth.desktop.kiro.dev/refreshToken",
     "xai": "https://auth.x.ai/oauth2/token",
@@ -49,7 +44,6 @@
   "clientIds": {
     "claude": "9d1c250a-e61b-44d9-88ed-5944d1962f5e",
     "codex": "app_EMoamEEZ73f0CkXaXp7hrann",
-    "qwen": "f0304373b74a44d2b584a3fb70ca9e56",
     "iflow": "10009311001",
     "kimi": "17e5f671-d194-4dfb-9706-5516cb48c098",
     "grok-cli": "b1a00492-073a-47ea-816f-4c329264a828"