Perf improvement: json_middleware

[pr-benchmarks]

Scenario	Environment	Date	Old RPS	New RPS	Change	Deviation	StDev	Dependencies
json_middleware [Fixed]	Json 10k 14- Trends 28 Core Cobalt Hosted Lin Server	12/02/2025 06:04:16	287,462	310,696	8.08 % (23,234) 👍	5.32 σ	4,369.68	Changes Name Version Diff Json.dll 1.0.0 -> 1.0.0 aspnet/Benchmarks@a464ca7...e907eb5

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@sebastienros