Revert "merge: tier2/phase2_4_5_call_site_completion_20260621 (parent + follow-up + Phase 6e analysis)"

This reverts commit f914b2bcd4, reversing
changes made to 7fef95cc87.

tests/test_ai_client_tool_loop.py

@@ -26,10 +26,10 @@ def caps() -> VendorCapabilities:
  return VendorCapabilities(vendor="test", model="test-model", tool_calling=True, context_window=8192)
 
 def _make_normalized_response(text: str = "ok", tool_calls: list[dict[str, Any]] | None = None) -> Result[NormalizedResponse]:
- from src.openai_schemas import UsageStats
  return Result(data=NormalizedResponse(
-  text=text, tool_calls=tool_calls or (),
-  usage=UsageStats(input_tokens=10, output_tokens=5),
+  text=text, tool_calls=tool_calls or [],
+  usage_input_tokens=10, usage_output_tokens=5,
+  usage_cache_read_tokens=0, usage_cache_creation_tokens=0,
   raw_response=None,
  ))