feat(qwen): add Qwen OAuth provider with portal request support

Based on #6079 by @tunamitom with critical fixes and comprehensive tests.

Changes from #6079:
- Fix: sanitization overwrite bug — Qwen message prep now runs AFTER codex
  field sanitization, not before (was silently discarding Qwen transforms)
- Fix: missing try/except AuthError in runtime_provider.py — stale Qwen
  credentials now fall through to next provider on auto-detect
- Fix: 'qwen' alias conflict — bare 'qwen' stays mapped to 'alibaba'
  (DashScope); use 'qwen-portal' or 'qwen-cli' for the OAuth provider
- Fix: hardcoded ['coder-model'] replaced with live API fetch + curated
  fallback list (qwen3-coder-plus, qwen3-coder)
- Fix: extract _is_qwen_portal() helper + _qwen_portal_headers() to replace
  5 inline 'portal.qwen.ai' string checks and share headers between init
  and credential swap
- Fix: add Qwen branch to _apply_client_headers_for_base_url for mid-session
  credential swaps
- Fix: remove suspicious TypeError catch blocks around _prompt_provider_choice
- Fix: handle bare string items in content lists (were silently dropped)
- Fix: remove redundant dict() copies after deepcopy in message prep
- Revert: unrelated ai-gateway test mock removal and model_switch.py comment deletion

New tests (30 test functions):
- _qwen_cli_auth_path, _read_qwen_cli_tokens (success + 3 error paths)
- _save_qwen_cli_tokens (roundtrip, parent creation, permissions)
- _qwen_access_token_is_expiring (5 edge cases: fresh, expired, within skew,
  None, non-numeric)
- _refresh_qwen_cli_tokens (success, preserve old refresh, 4 error paths,
  default expires_in, disk persistence)
- resolve_qwen_runtime_credentials (fresh, auto-refresh, force-refresh,
  missing token, env override)
- get_qwen_auth_status (logged in, not logged in)
- Runtime provider resolution (direct, pool entry, alias)
- _build_api_kwargs (metadata, vl_high_resolution_images, message formatting,
  max_tokens suppression)

hermes_cli/providers.py

@@ -58,6 +58,12 @@ HERMES_OVERLAYS: Dict[str, HermesOverlay] = {
         auth_type="oauth_external",
         base_url_override="https://chatgpt.com/backend-api/codex",
     ),
+    "qwen-oauth": HermesOverlay(
+        transport="openai_chat",
+        auth_type="oauth_external",
+        base_url_override="https://portal.qwen.ai/v1",
+        base_url_env_var="HERMES_QWEN_BASE_URL",
+    ),
     "copilot-acp": HermesOverlay(
         transport="codex_responses",
         auth_type="external_process",