feat: add APIMODELS as an OpenAI-compatible provider

APIMODELS (https://apimodels.app) is an OpenAI-compatible gateway. Registered
through the JSON provider system, plus the four registration points a JSON
provider needs to be fully usable:

- litellm/llms/openai_like/providers.json  - base_url, api_key_env, api_base_env,
  supported_endpoints (/v1/chat/completions and /v1/responses)
- litellm/types/utils.py                   - LlmProviders.APIMODELS
- litellm/constants.py                     - openai_compatible_providers and
                                             openai_compatible_endpoints
- provider_create_fields.json              - selectable in the proxy UI

Note on openai_compatible_endpoints: the contributing guide only mentions
providers.json, but api_base auto-detection in get_llm_provider_logic iterates
litellm.openai_compatible_endpoints, so a provider missing from that list is
resolvable by the apimodels/<model> prefix yet silently unresolvable when only
api_base is supplied. Adding it there keeps both paths working.

Tests: tests/test_litellm/llms/openai_like/test_apimodels_provider.py
9 tests, configuration and mock assertions only, no network calls.

tests/unit/llms/openai_like/test_apimodels_provider.py

@@ -0,0 +1,128 @@
+"""
+Tests for the APIMODELS provider configuration.
+
+APIMODELS (https://apimodels.app) is an OpenAI-compatible gateway, so it is
+registered through the JSON provider system rather than a bespoke Python module.
+These tests are pure configuration assertions and mocks — no network calls.
+"""
+
+import litellm
+
+
+class TestAPIModelsProviderConfig:
+    def test_apimodels_in_provider_list(self):
+        from litellm import LlmProviders
+
+        assert hasattr(LlmProviders, "APIMODELS")
+        assert LlmProviders.APIMODELS.value == "apimodels"
+        assert "apimodels" in litellm.provider_list
+
+    def test_apimodels_json_config_exists(self):
+        from litellm.llms.openai_like.json_loader import JSONProviderRegistry
+
+        assert JSONProviderRegistry.exists("apimodels")
+
+        provider = JSONProviderRegistry.get("apimodels")
+        assert provider is not None
+        assert provider.base_url == "https://api.apimodels.app/v1"
+        assert provider.api_key_env == "APIMODELS_API_KEY"
+
+    def test_apimodels_in_openai_compatible_providers(self):
+        from litellm.constants import openai_compatible_providers
+
+        assert "apimodels" in openai_compatible_providers
+
+    def test_apimodels_supports_responses_api(self):
+        from litellm.llms.openai_like.json_loader import JSONProviderRegistry
+
+        assert JSONProviderRegistry.supports_responses_api("apimodels")
+
+    def test_apimodels_provider_resolution(self):
+        from litellm.litellm_core_utils.get_llm_provider_logic import get_llm_provider
+
+        model, provider, api_key, api_base = get_llm_provider(
+            model="apimodels/gpt-5.6-luna",
+            custom_llm_provider=None,
+            api_base=None,
+            api_key=None,
+        )
+
+        assert model == "gpt-5.6-luna"
+        assert provider == "apimodels"
+        assert api_base == "https://api.apimodels.app/v1"
+
+    def test_apimodels_api_base_override(self):
+        from litellm.litellm_core_utils.get_llm_provider_logic import get_llm_provider
+
+        model, provider, api_key, api_base = get_llm_provider(
+            model="apimodels/gpt-5.6-luna",
+            custom_llm_provider=None,
+            api_base="https://custom.apimodels.app/v1",
+            api_key="sk-test",
+        )
+
+        assert provider == "apimodels"
+        assert api_base == "https://custom.apimodels.app/v1"
+        assert api_key == "sk-test"
+
+    def test_apimodels_url_autodetection(self):
+        from litellm.litellm_core_utils.get_llm_provider_logic import get_llm_provider
+
+        model, provider, api_key, api_base = get_llm_provider(
+            model="gpt-5.6-luna",
+            custom_llm_provider=None,
+            api_base="https://api.apimodels.app/v1",
+            api_key=None,
+        )
+
+        assert provider == "apimodels"
+        assert api_base == "https://api.apimodels.app/v1"
+
+    def test_apimodels_router_config(self):
+        from litellm import Router
+
+        router = Router(
+            model_list=[
+                {
+                    "model_name": "apimodels-chat",
+                    "litellm_params": {
+                        "model": "apimodels/gpt-5.6-luna",
+                        "api_key": "test-key",
+                    },
+                }
+            ]
+        )
+
+        assert len(router.model_list) == 1
+        assert router.model_list[0]["model_name"] == "apimodels-chat"
+
+
+class TestAPIModelsDashboardRegistration:
+    @staticmethod
+    def _provider_create_fields():
+        import json
+        from pathlib import Path
+
+        path = (
+            Path(litellm.__file__).parent
+            / "proxy"
+            / "public_endpoints"
+            / "provider_create_fields.json"
+        )
+        with open(path) as f:
+            return json.load(f)
+
+    def test_apimodels_is_selectable_in_the_add_model_form(self):
+        entries = [
+            e for e in self._provider_create_fields() if e["litellm_provider"] == "apimodels"
+        ]
+        assert len(entries) == 1, "apimodels must appear exactly once in provider_create_fields.json"
+
+        entry = entries[0]
+        assert entry["provider"] == "APIMODELS"
+        assert entry["default_model_placeholder"].startswith("apimodels/")
+
+        fields = {f["key"]: f for f in entry["credential_fields"]}
+        assert fields["api_key"]["required"] is True
+        assert fields["api_key"]["field_type"] == "password"
+        assert fields["api_base"]["required"] is False