refactor(providers): simplify cached_tokens extraction with _get_nested_int

Extract a _get_nested_int helper that unifies dict-key and attribute
access, then express the 3-tier provider fallback as a simple loop
over path tuples instead of duplicated if/else chains.

nanobot/providers/openai_compat_provider.py

@@ -308,6 +308,13 @@ class OpenAICompatProvider(LLMProvider):
 
     @classmethod
     def _extract_usage(cls, response: Any) -> dict[str, int]:
+        """Extract token usage from an OpenAI-compatible response.
+
+        Handles both dict-based (raw JSON) and object-based (SDK Pydantic)
+        responses.  Provider-specific ``cached_tokens`` fields are normalised
+        under a single key; see the priority chain inside for details.
+        """
+        # --- resolve usage object ---
         usage_obj = None
         response_map = cls._maybe_mapping(response)
         if response_map is not None:
@@ -331,34 +338,40 @@ class OpenAICompatProvider(LLMProvider):
         else:
             return {}
 
-        # Extract cached_tokens from various provider formats.
-        # Priority: prompt_tokens_details > top-level cached_tokens > prompt_cache_hit_tokens
-        cached = 0
-        # 1. OpenAI / Zhipu / MiniMax / Qwen / SiliconFlow / 豆包 / Mistral / xAI:
-        #    nested prompt_tokens_details.cached_tokens
-        details = (usage_map or {}).get("prompt_tokens_details") if usage_map else None
-        if not cls._maybe_mapping(details):
-            details = getattr(usage_obj, "prompt_tokens_details", None) if usage_obj else None
-        details_map = cls._maybe_mapping(details)
-        if details_map is not None:
-            cached = int(details_map.get("cached_tokens") or 0)
-        elif details is not None:
-            cached = int(getattr(details, "cached_tokens", 0) or 0)
-        # 2. StepFun / Moonshot: top-level usage.cached_tokens
-        if not cached and usage_map is not None:
-            cached = int(usage_map.get("cached_tokens") or 0)
-        if not cached and usage_obj and not usage_map:
-            cached = int(getattr(usage_obj, "cached_tokens", 0) or 0)
-        # 3. DeepSeek / SiliconFlow extra: top-level prompt_cache_hit_tokens
-        if not cached and usage_map is not None:
-            cached = int(usage_map.get("prompt_cache_hit_tokens") or 0)
-        if not cached and usage_obj and not usage_map:
-            cached = int(getattr(usage_obj, "prompt_cache_hit_tokens", 0) or 0)
-        if cached:
-            result["cached_tokens"] = cached
+        # --- cached_tokens (normalised across providers) ---
+        # Try nested paths first (dict), fall back to attribute (SDK object).
+        # Priority order ensures the most specific field wins.
+        for path in (
+            ("prompt_tokens_details", "cached_tokens"),  # OpenAI/Zhipu/MiniMax/Qwen/Mistral/xAI
+            ("cached_tokens",),                          # StepFun/Moonshot (top-level)
+            ("prompt_cache_hit_tokens",),                # DeepSeek/SiliconFlow
+        ):
+            cached = cls._get_nested_int(usage_map, path)
+            if not cached and usage_obj:
+                cached = cls._get_nested_int(usage_obj, path)
+            if cached:
+                result["cached_tokens"] = cached
+                break
 
         return result
 
+    @staticmethod
+    def _get_nested_int(obj: Any, path: tuple[str, ...]) -> int:
+        """Drill into *obj* by *path* segments and return an ``int`` value.
+
+        Supports both dict-key access and attribute access so it works
+        uniformly with raw JSON dicts **and** SDK Pydantic models.
+        """
+        current = obj
+        for segment in path:
+            if current is None:
+                return 0
+            if isinstance(current, dict):
+                current = current.get(segment)
+            else:
+                current = getattr(current, segment, None)
+        return int(current or 0) if current is not None else 0
+
     def _parse(self, response: Any) -> LLMResponse:
         if isinstance(response, str):
             return LLMResponse(content=response, finish_reason="stop")