feat: add Vercel AI Gateway provider (#1628)

* feat: add Vercel AI Gateway as a first-class provider

Adds AI Gateway (ai-gateway.vercel.sh) as a new inference provider
with AI_GATEWAY_API_KEY authentication, live model discovery, and
reasoning support via extra_body.reasoning.

Based on PR #1492 by jerilynzheng.

* feat: add AI Gateway to setup wizard, doctor, and fallback providers

* test: add AI Gateway to api_key_providers test suite

* feat: add AI Gateway to hermes model CLI and model metadata

Wire AI Gateway into the interactive model selection menu and add
context lengths for AI Gateway model IDs in model_metadata.py.

* feat: use claude-haiku-4.5 as AI Gateway auxiliary model

* revert: use gemini-3-flash as AI Gateway auxiliary model

* fix: move AI Gateway below established providers in selection order

---------

Co-authored-by: jerilynzheng <jerilynzheng@users.noreply.github.com>
Co-authored-by: jerilynzheng <zheng.jerilyn@gmail.com>

run_agent.py

@@ -3523,6 +3523,8 @@ class AIAgent:
         base_url = (self.base_url or "").lower()
         if "nousresearch" in base_url:
             return True
+        if "ai-gateway.vercel.sh" in base_url:
+            return True
         if "openrouter" not in base_url:
             return False
         if "api.mistral.ai" in base_url: