refactor(reasoning_effort_grid): tighten test helpers per Greptile review

Two P2 nits flagged by Greptile on PR 28036:

1. _build_completion_kwargs() defaulted vertex_project to "vertex-check-481318"
   when VERTEX_PROJECT was unset. That value is a specific GCP project that
   doesn't belong to this repo, so if the env-var skip guard were ever
   bypassed (misconfig, direct helper call), the test would silently issue
   calls to a foreign project rather than failing loudly. Drop the fallback
   and read os.environ["VERTEX_PROJECT"] directly, mirroring how
   AZURE_FOUNDRY_* are handled.

2. _build_messages_kwargs() was a one-liner that returned the result of
   _build_completion_kwargs() unchanged -- a dead abstraction with one
   caller. Inline at the _call_messages call site and delete the helper.

tests/llm_translation/reasoning_effort_grid/test_reasoning_effort_grid.py

@@ -65,20 +65,13 @@ def _build_completion_kwargs(model: ModelEntry, effort: str) -> Dict[str, Any]:
     if effort != "__omit__":
         kwargs["reasoning_effort"] = effort
     if model.model.startswith("vertex_ai/"):
-        kwargs["vertex_project"] = os.environ.get(
-            "VERTEX_PROJECT", "vertex-check-481318"
-        )
+        kwargs["vertex_project"] = os.environ["VERTEX_PROJECT"]
     if model.model.startswith("azure_ai/"):
         kwargs["api_base"] = os.environ["AZURE_FOUNDRY_API_BASE"]
         kwargs["api_key"] = os.environ["AZURE_FOUNDRY_API_KEY"]
     return kwargs
 
 
-def _build_messages_kwargs(model: ModelEntry, effort: str) -> Dict[str, Any]:
-    kwargs = _build_completion_kwargs(model, effort)
-    return kwargs
-
-
 def _converse_subbody(body: Dict[str, Any]) -> Dict[str, Any]:
     """Return the dict that holds thinking/output_config for a Converse wire body."""
     return body.get("additionalModelRequestFields", body)
@@ -185,7 +178,7 @@ async def _call_chat(model: ModelEntry, effort: str) -> Tuple[int, Optional[Exce
 async def _call_messages(
     model: ModelEntry, effort: str
 ) -> Tuple[int, Optional[Exception]]:
-    kwargs = _build_messages_kwargs(model, effort)
+    kwargs = _build_completion_kwargs(model, effort)
     try:
         await litellm.anthropic_messages(**kwargs)
         return 200, None