refactor(providers): define typed usage contract

tests/webui/test_session_context.py

@@ -1,3 +1,4 @@
+from nanobot.providers.base import LLMUsage
 from nanobot.session import Session
 from nanobot.utils.helpers import estimate_message_tokens
 from nanobot.webui.session_context import session_context_payload
@@ -58,19 +59,12 @@ def test_session_context_tolerates_untrusted_summary_metadata() -> None:
 
 
 def test_session_context_sanitizes_usage_metadata() -> None:
+    usage = LLMUsage.reported(input_tokens=120, output_tokens=8, total_tokens=175)
     session = Session(
         key="websocket:context",
-        metadata={
-            "_last_usage": {
-                "prompt_tokens": 120,
-                "completion_tokens": 8,
-                "negative": -1,
-                "boolean": True,
-                "text": "invalid",
-            }
-        },
+        metadata={"_last_usage": usage.to_dict()},
     )
 
     payload = session_context_payload(session)
 
-    assert payload["last_usage"] == {"prompt_tokens": 120, "completion_tokens": 8}
+    assert payload["last_usage"] == usage.to_dict()