Perf improvement: json_middleware

[pr-benchmarks]

Scenario	Environment	Date	Old RPS	New RPS	Change	Deviation	StDev	Dependencies
json_middleware [Fixed]	Json 10k 2- Trends Cobalt Cloud Linux	07/08/2025 09:44:20	174,348	174,510	0.09 % (162) 👍	2.29 σ	70.76	Changes Name Version Diff Json.dll 1.0.0 -> 1.0.0 aspnet/Benchmarks@670c50d...aa7092a

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