refactor(providers): define typed usage contract

nanobot/agent/turn_delivery.py

@@ -4,7 +4,7 @@ from __future__ import annotations
 
 import dataclasses
 import time
-from collections.abc import Awaitable, Callable, Mapping
+from collections.abc import Awaitable, Callable
 from dataclasses import dataclass, field
 from typing import TYPE_CHECKING, Any, cast
 
@@ -19,6 +19,7 @@ from nanobot.bus.outbound_events import (
 from nanobot.bus.progress import build_bus_progress_callback
 from nanobot.bus.queue import MessageBus
 from nanobot.bus.runtime_events import RuntimeEventBus, RuntimeEventPublisher
+from nanobot.providers.base import LLMUsage
 
 if TYPE_CHECKING:
     from nanobot.utils.llm_runtime import LLMRuntime
@@ -203,7 +204,7 @@ class TurnDelivery:
     def record_latency(self, latency_ms: int | None) -> None:
         self.runtime_event_publisher.record_turn_latency(self.session_key, latency_ms)
 
-    def record_usage(self, usage: Mapping[str, int]) -> None:
+    def record_usage(self, usage: LLMUsage | None) -> None:
         self.runtime_event_publisher.record_turn_usage(self.session_key, usage)
 
     def background_response(