chore(types): annotate four containers mypy could not infer

Each element type is taken from how the container is used rather than
guessed: kept_items is returned from trim_to_fit, whose signature is
already list[Any]; traces collects the dicts built at the append site;
expert_results and tool_outputs are keyed by tool_name (str) and hold
ToolReturnPart.content.

tracing_router.py needed `from typing import Any` added — it had no
import for it, so annotating without that would have traded a
var-annotated error for a name-defined one. Function-body variable
annotations are not evaluated at runtime, so this would not have raised;
mypy was the only thing that would have caught it.

90 errors -> 86.

Co-Authored-By: Claude <noreply@anthropic.com>

src/responses/context.py

@@ -83,7 +83,7 @@ class ContextWindow:
             return []
 
         # Start from most recent, work backwards
-        kept_items = []
+        kept_items: list[Any] = []
         current_tokens = 0
 
         for item in reversed(items):