Perf improvement: json_middleware

[pr-benchmarks]

Scenario	Environment	Date	Old RPS	New RPS	Change	Deviation	StDev	Dependencies
json_middleware	Json 10k 6- Trends Intel Win	07/31/2025 12:46:59	386,795	403,773	4.39 % (16,977) 👍	2.24 σ	7,565.14	Changes Name Version Diff Json.dll 1.0.0 -> 1.0.0 aspnet/Benchmarks@4f899da...a05388c

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