refactor(test): validate cost-map entries into a typed model

The selector read raw cost-map dicts as `Mapping[str, Any]`. It now validates
each together_ai entry into a frozen Pydantic model and takes the two
capabilities as keyword booleans, so nothing in the helper is coarsely typed
or stringly addressed.

tests/_live_test_helpers.py

@@ -1,9 +1,8 @@
 import os
-from collections.abc import Mapping
 from datetime import date
-from typing import Any
 
 import pytest
+from pydantic import BaseModel, ConfigDict
 
 
 def _skip_live_prompt_caching_test():
@@ -13,34 +12,53 @@ def _skip_live_prompt_caching_test():
         pytest.skip("Live prompt-caching E2E tests cannot run under VCR replay")
 
 
-def cheapest_together_chat_model(*capability_flags: str) -> str:
+
+class TogetherCostEntry(BaseModel):
+    model_config = ConfigDict(frozen=True, extra="ignore")
+
+    litellm_provider: str | None = None
+    mode: str | None = None
+    deprecation_date: str | None = None
+    input_cost_per_token: float | None = None
+    output_cost_per_token: float | None = None
+    supports_function_calling: bool | None = None
+    supports_response_schema: bool | None = None
+
+
+def cheapest_together_chat_model(
+    *, function_calling: bool = False, response_schema: bool = False
+) -> str:
     import litellm
 
     today = date.today().isoformat()
 
-    def qualifies(name: str, entry: Mapping[str, Any]) -> bool:
-        deprecation_date = entry.get("deprecation_date")
+    def qualifies(name: str, entry: TogetherCostEntry) -> bool:
         return (
             name.startswith("together_ai/")
-            and entry.get("litellm_provider") == "together_ai"
-            and entry.get("mode") == "chat"
-            and (deprecation_date is None or deprecation_date > today)
-            and (entry.get("input_cost_per_token") or 0.0) > 0
-            and (entry.get("output_cost_per_token") or 0.0) > 0
-            and all(bool(entry.get(flag)) for flag in capability_flags)
+            and entry.litellm_provider == "together_ai"
+            and entry.mode == "chat"
+            and (entry.deprecation_date is None or entry.deprecation_date > today)
+            and (entry.input_cost_per_token or 0.0) > 0
+            and (entry.output_cost_per_token or 0.0) > 0
+            and (not function_calling or bool(entry.supports_function_calling))
+            and (not response_schema or bool(entry.supports_response_schema))
         )
 
+    registry: dict[str, TogetherCostEntry] = {
+        name: TogetherCostEntry.model_validate(raw)
+        for name, raw in litellm.model_cost.items()
+        if isinstance(raw, dict) and name.startswith("together_ai/")
+    }
     candidates = sorted(
-        (
-            name
-            for name, entry in litellm.model_cost.items()
-            if isinstance(entry, Mapping) and qualifies(name, entry)
-        ),
+        (name for name, entry in registry.items() if qualifies(name, entry)),
         key=lambda name: (
-            litellm.model_cost[name].get("input_cost_per_token") or 0.0,
-            litellm.model_cost[name].get("output_cost_per_token") or 0.0,
+            registry[name].input_cost_per_token or 0.0,
+            registry[name].output_cost_per_token or 0.0,
             name,
         ),
     )
-    assert candidates, f"no live together_ai chat model in the cost map satisfies {capability_flags}"
+    assert candidates, (
+        "no live together_ai chat model in the cost map satisfies "
+        f"function_calling={function_calling} response_schema={response_schema}"
+    )
     return candidates[0]

tests/llm_translation/test_together_ai.py

@@ -19,7 +19,7 @@ class TestTogetherAI(BaseLLMChatTest):
         litellm.set_verbose = True
         return {
             "model": cheapest_together_chat_model(
-                "supports_function_calling", "supports_response_schema"
+                function_calling=True, response_schema=True
             )
         }