fix(responses): prioritize Python classes over JSON in get_provider_chat_config

- Invert lookup order in get_provider_chat_config so Python classes
  (with custom overrides) are checked before JSON providers, matching
  the pattern already used in get_provider_responses_api_config.
  Prevents regression where providers like Perplexity (declared in
  providers.json) would silently lose their custom chat config.
- Add 'models' to Perplexity Responses API supported params (fallback
  chain feature documented in Perplexity API).

litellm/llms/perplexity/responses/transformation.py

@@ -29,6 +29,7 @@ class PerplexityResponsesConfig(OpenAIResponsesAPIConfig):
             "tools",
             "reasoning",
             "instructions",
+            "models",
         ]
 
     @property

litellm/utils.py

@@ -8060,17 +8060,8 @@ class ProviderConfigManager:
         Returns the provider config for a given provider.
 
         Uses O(1) dictionary lookup for fast provider resolution.
+        Python classes take priority over JSON (they have custom overrides).
         """
-        # Check JSON providers FIRST (these override standard mappings)
-        from litellm.llms.openai_like.dynamic_config import create_config_class
-        from litellm.llms.openai_like.json_loader import JSONProviderRegistry
-
-        if JSONProviderRegistry.exists(provider.value):
-            provider_config = JSONProviderRegistry.get(provider.value)
-            if provider_config is None:
-                raise ValueError(f"Provider {provider.value} not found")
-            return create_config_class(provider_config)()
-
         # Handle OpenAI special cases (O-series and GPT-5 models)
         if provider == LlmProviders.OPENAI:
             if litellm.openaiOSeriesConfig.is_model_o_series_model(model=model):
@@ -8084,18 +8075,24 @@ class ProviderConfigManager:
                 ProviderConfigManager._build_provider_config_map()
             )
 
-        # O(1) dictionary lookup
+        # O(1) dictionary lookup — Python classes first (custom overrides take priority)
         config_entry = ProviderConfigManager._PROVIDER_CONFIG_MAP.get(provider)
-        if config_entry is None:
-            return None
+        if config_entry is not None:
+            config_factory, needs_model = config_entry
+            if needs_model:
+                return config_factory(model)  # type: ignore
+            else:
+                return config_factory()  # type: ignore
 
-        # Unpack factory function and whether it needs model parameter
-        # This avoids expensive inspect.signature() calls at runtime
-        config_factory, needs_model = config_entry
-        if needs_model:
-            return config_factory(model)  # type: ignore
-        else:
-            return config_factory()  # type: ignore
+        # Fall back to JSON providers (generic OpenAI-compatible)
+        from litellm.llms.openai_like.dynamic_config import create_config_class
+        from litellm.llms.openai_like.json_loader import JSONProviderRegistry
+
+        if JSONProviderRegistry.exists(provider.value):
+            provider_config = JSONProviderRegistry.get(provider.value)
+            if provider_config is None:
+                raise ValueError(f"Provider {provider.value} not found")
+            return create_config_class(provider_config)()
 
     @staticmethod
     def get_provider_embedding_config(