http: enforce request body size limit to prevent DoS attacks

[KambojRajan]

Add request body size enforcement in accessController.ServeHTTP() in server/embed/serve.go, which is the entry point for all HTTP requests before they reach the gRPC-gateway handlers.
Two layers of protection:

Fast-reject via Content-Length: If the declared Content-Length exceeds MaxRequestBytesWithOverhead(), return 413 Request Entity Too Large immediately without reading any body bytes.
Safety net via http.MaxBytesReader: Wraps req.Body so the JSON decoder cannot read past the con

[k8s-ci-robot]

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This pull-request has been approved by: KambojRajan
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[k8s-ci-robot]

Hi @KambojRajan. Thanks for your PR.

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