fix(types): widen UsageNormalizer signatures to match actual runtime behavior

- Add UsageSummary to normalize() parameter type (was handled at runtime
  but not declared in the signature)
- Change merge_streaming_usage() params to accept None and dict[str, Any]
  (was handling None at runtime but typed as dict[str, int])
- Change merge_streaming_usage() return type to dict[str, Any] to reflect
  that it preserves nested dicts (completion_tokens_details, cost, etc.)
- Remove unreachable isinstance(usage, dict) guard after type narrowing

Author: Mateusz

src/core/transport/fastapi/adapters/usage/normalizer.py

@@ -7,6 +7,7 @@
 
 if TYPE_CHECKING:
     from src.core.domain.openrouter_usage import OpenRouterUsage
+    from src.core.domain.usage_summary import UsageSummary
     from src.core.services.usage_calculation_service import UsageCalculationService
 
 logger = logging.getLogger(__name__)
@@ -32,12 +33,13 @@ def __init__(
         self._usage_service = usage_service
 
     def normalize(
-        self, usage: dict[str, Any] | OpenRouterUsage | None
+        self,
+        usage: dict[str, Any] | OpenRouterUsage | UsageSummary | None,
     ) -> dict[str, int]:
         """Normalize usage to standard format.
 
         Args:
-            usage: Usage dictionary, OpenRouterUsage instance, or None
+            usage: Usage dictionary, OpenRouterUsage, UsageSummary, or None
 
         Returns:
             Normalized usage with standard fields as integers
@@ -61,13 +63,6 @@ def normalize(
         if isinstance(usage, UsageSummary):
             usage = usage.to_legacy_dict()
 
-        if not isinstance(usage, dict):
-            return {
-                "prompt_tokens": 0,
-                "completion_tokens": 0,
-                "total_tokens": 0,
-            }
-
         usage = dict(usage)
 
         if "prompt_tokens" not in usage and "input_tokens" in usage:
@@ -180,13 +175,15 @@ def _ensure_total_valid(self, usage: dict[str, Any]) -> dict[str, int]:
         return usage
 
     def merge_streaming_usage(
-        self, existing: dict[str, int], new: dict[str, Any]
-    ) -> dict[str, int]:
+        self,
+        existing: dict[str, Any] | None,
+        new: dict[str, Any] | None,
+    ) -> dict[str, Any]:
         """Merge usage keeping highest values.
 
         Args:
-            existing: Existing usage dictionary
-            new: New usage dictionary to merge
+            existing: Existing usage dictionary or None
+            new: New usage dictionary to merge or None
 
         Returns:
             Merged usage dictionary with highest values

tests/unit/transport/fastapi/adapters/usage/test_usage_normalizer.py

@@ -177,9 +177,7 @@ def test_responses_api_input_tokens_mapped_to_prompt_tokens(self):
     def test_responses_api_only_output_tokens_mapped(self):
         """If only output_tokens present (no prompt_tokens), it maps correctly."""
         normalizer = UsageNormalizer()
-        result = normalizer.normalize(
-            {"output_tokens": 15, "total_tokens": 57}
-        )
+        result = normalizer.normalize({"output_tokens": 15, "total_tokens": 57})
         assert result["prompt_tokens"] == 0
         assert result["completion_tokens"] == 15
         assert result["total_tokens"] == 57