style: bring the savings changes under the strict and type-discipline budgets

`Optional[PreRoutingHookResponse]` becomes the union form the codebase is moving
to, the recorded metadata is built in one shot rather than seeded empty and
mutated, and the spend log's usage object is typed as a Mapping of object rather
than a bare mutable dict, which also drops the Any the strict gate bans.

The local run compared against a stale base and read clean, so these only
surfaced once CI diffed against the real merge base.

litellm/proxy/spend_tracking/savings.py

@@ -8,6 +8,7 @@ are known) and summed into the daily tables; tokens cannot be priced after they
 have been aggregated across models.
 """
 
+from collections.abc import Mapping
 from typing import NamedTuple
 
 import litellm
@@ -172,7 +173,7 @@ def compute_autorouter_savings(
     return baseline_cost - selected_cost
 
 
-def _usage_from_spend_log(usage_object: dict | None) -> Usage | None:
+def _usage_from_spend_log(usage_object: Mapping[str, object] | None) -> Usage | None:
     """Rebuild the request's ``Usage`` from the copy the spend log recorded."""
     if not usage_object:
         return None
@@ -192,7 +193,7 @@ def compute_savings_spend(
     compression_saved_tokens: int,
     cache_read_input_tokens: int,
     baseline_model: str | None = None,
-    usage_object: dict | None = None,
+    usage_object: Mapping[str, object] | None = None,
 ) -> SavingsSpend:
     """
     Dollar savings for one request, split by optimization driver.

litellm/router.py

@@ -11191,7 +11191,7 @@ class Router:
     @staticmethod
     def _record_routing_decision(
         request_kwargs: dict,
-        pre_routing_hook_response: Optional[PreRoutingHookResponse],
+        pre_routing_hook_response: PreRoutingHookResponse | None,
     ) -> None:
         """Make the request's metadata describe THIS routing attempt, and only this one.
 
@@ -11210,15 +11210,23 @@ class Router:
         routing_decision = pre_routing_hook_response.routing_decision if pre_routing_hook_response else None
         baseline_model = pre_routing_hook_response.savings_baseline_model if pre_routing_hook_response else None
 
-        recorded: dict[str, Any] = {}
-        if routing_decision is not None:
-            recorded["routing_decision"] = Router._redact_prompt_text_if_needed(
-                request_kwargs=request_kwargs, routing_decision=routing_decision
+        recorded = {
+            key: value
+            for key, value in (
+                (
+                    "routing_decision",
+                    Router._redact_prompt_text_if_needed(
+                        request_kwargs=request_kwargs, routing_decision=routing_decision
+                    )
+                    if routing_decision is not None
+                    else None,
+                ),
+                ("auto_router_savings_baseline_model", baseline_model),
             )
-        if baseline_model is not None:
-            recorded["auto_router_savings_baseline_model"] = baseline_model
+            if value is not None
+        }
 
-        cleared = {"routing_decision", "auto_router_savings_baseline_model"} - recorded.keys()
+        cleared = frozenset({"routing_decision", "auto_router_savings_baseline_model"}) - recorded.keys()
         for bucket in (request_kwargs.get("metadata"), request_kwargs.get("litellm_metadata")):
             if isinstance(bucket, dict):
                 for key in cleared: