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>

hermes_cli/auth.py

@@ -155,6 +155,14 @@ PROVIDER_REGISTRY: Dict[str, ProviderConfig] = {
         api_key_env_vars=("DEEPSEEK_API_KEY",),
         base_url_env_var="DEEPSEEK_BASE_URL",
     ),
+    "ai-gateway": ProviderConfig(
+        id="ai-gateway",
+        name="AI Gateway",
+        auth_type="api_key",
+        inference_base_url="https://ai-gateway.vercel.sh/v1",
+        api_key_env_vars=("AI_GATEWAY_API_KEY",),
+        base_url_env_var="AI_GATEWAY_BASE_URL",
+    ),
 }
 
 
@@ -532,6 +540,7 @@ def resolve_provider(
         "kimi": "kimi-coding", "moonshot": "kimi-coding",
         "minimax-china": "minimax-cn", "minimax_cn": "minimax-cn",
         "claude": "anthropic", "claude-code": "anthropic",
+        "aigateway": "ai-gateway", "vercel": "ai-gateway", "vercel-ai-gateway": "ai-gateway",
     }
     normalized = _PROVIDER_ALIASES.get(normalized, normalized)