refactor(providers): define typed usage contract

tests/utils/test_searchusage.py

@@ -6,6 +6,7 @@ from unittest.mock import AsyncMock, MagicMock, patch
 
 import pytest
 
+from nanobot.providers.base import LLMUsage
 from nanobot.utils.helpers import build_status_content
 from nanobot.utils.searchusage import (
     SearchUsageInfo,
@@ -273,7 +274,7 @@ class TestBuildStatusContentWithSearchUsage:
         version="0.1.0",
         model="claude-opus-4-5",
         start_time=1_000_000.0,
-        last_usage={"prompt_tokens": 1000, "completion_tokens": 200},
+        last_usage=LLMUsage.reported(input_tokens=1000, output_tokens=200),
         context_window_tokens=65536,
         session_msg_count=5,
         context_tokens_estimate=3000,