fix(mcp): reuse proxy_logging DualCache in inject_mcp_jwt_headers_for_upstream

Instead of allocating a fresh DualCache() on every tools/list invocation,
prefer the shared proxy_logging_obj.internal_usage_cache.dual_cache when
available. The cache argument is currently unused by MCPJWTSigner, but
sharing the proxy's cache avoids per-call allocation overhead and matches
the cache identity used elsewhere in the proxy hook plumbing — so any
future per-request state stored in cache will survive across list calls.

Co-authored-by: Claude <noreply@anthropic.com>

litellm/proxy/guardrails/guardrail_hooks/mcp_jwt_signer/mcp_jwt_signer.py

@@ -945,9 +945,22 @@ async def inject_mcp_jwt_headers_for_upstream(
     call_type: CallTypesLiteral = (
         "list_mcp_tools" if for_list_tools else "call_mcp_tool"
     )
+    try:
+        from litellm.proxy.proxy_server import (  # noqa: PLC0415
+            proxy_logging_obj as _proxy_logging,
+        )
+
+        shared_cache = (
+            _proxy_logging.internal_usage_cache.dual_cache
+            if _proxy_logging is not None
+            else DualCache()
+        )
+    except Exception:
+        shared_cache = DualCache()
+
     result = await signer.async_pre_call_hook(
         user_api_key_dict=user_api_key_dict,
-        cache=DualCache(),
+        cache=shared_cache,
         data=hook_data,
         call_type=call_type,
     )