test(token_usage): migrate to send_result() (Phase 2.6)

The test_token_usage_tracking test calls ai_client.send() and verifies
the comms log entry. Migrating to send_result() + assert result.ok.

Changes:
- Rename ai_client.send(...) to ai_client.send_result(...)
- Add assert result.ok
- Add "from src.result_types import Result" import

1 test passes.

tests/test_token_usage.py

@@ -6,6 +6,7 @@ from types import SimpleNamespace
 sys.path.append(os.path.abspath(os.path.join(os.path.dirname(__file__), "..")))
 
 from src import ai_client
+from src.result_types import Result
 
 def test_token_usage_tracking() -> None:
  ai_client.reset_session()
@@ -31,7 +32,8 @@ def test_token_usage_tracking() -> None:
   mock_response.text = "Mock Response"
   mock_chat.send_message.return_value = mock_response
   ai_client.set_provider("gemini", "gemini-2.5-flash-lite")
-  ai_client.send("Context", "Hello")
+  result = ai_client.send_result("Context", "Hello")
+  assert result.ok
   comms = ai_client.get_comms_log()
   response_entries = [e for e in comms if e.get("direction") == "IN" and e["kind"] == "response"]
   assert len(response_entries) > 0