refactor(agent): make run usage explicit (#5546)

* refactor(agent): make run usage explicit

* fix(api): capture usage per run

tests/agent/test_hook_composite.py

@@ -457,12 +457,12 @@ async def test_agent_loop_extra_hook_receives_calls(tmp_path):
     )
     loop.tools.get_definitions = MagicMock(return_value=[])
 
-    content, tools_used, messages, _, _ = await loop._run_agent_loop(
+    result = await loop._run_agent_loop(
         [{"role": "user", "content": "hi"}],
         runtime=loop.llm_runtime(),
     )
 
-    assert content == "done"
+    assert result.final_content == "done"
     assert "before_run" in events
     assert "before_iter:0" in events
     assert "after_iter:0" in events
@@ -545,12 +545,12 @@ async def test_agent_loop_extra_hook_error_isolation(tmp_path):
     )
     loop.tools.get_definitions = MagicMock(return_value=[])
 
-    content, _, _, _, _ = await loop._run_agent_loop(
+    result = await loop._run_agent_loop(
         [{"role": "user", "content": "hi"}],
         runtime=loop.llm_runtime(),
     )
 
-    assert content == "still works"
+    assert result.final_content == "still works"
 
 
 @pytest.mark.asyncio
@@ -590,11 +590,11 @@ async def test_agent_loop_no_hooks_backward_compat(tmp_path):
     loop.tools.execute = AsyncMock(return_value="ok")
     loop.max_iterations = 2
 
-    content, tools_used, _, _, _ = await loop._run_agent_loop(
+    result = await loop._run_agent_loop(
         [], runtime=loop.llm_runtime()
     )
-    assert content == (
+    assert result.final_content == (
         "I reached the maximum number of tool call iterations (2) "
         "without completing the task. You can try breaking the task into smaller steps."
     )
-    assert tools_used == ["list_dir", "list_dir"]
+    assert result.tools_used == ["list_dir", "list_dir"]