fix(router): prevent OOM by enforcing size limits on HTTP request bodies

[safiya2610]

Description

Fixes #1641

This PR addresses a critical Denial of Service (DoS) vulnerability where unbounded user requests could exhaust router memory and cause the pod to crash (OOMKilled).

Previously, several HTTP handlers used io.ReadAll directly on uncontrolled input streams without enforcing a maximum payload size. This PR mitigates the vulnerability by wrapping the incoming request bodies in an io.LimitReader configured with a strict 10MB limit before reading them into memory.

Changes Made

• pkg/kthena-router/router/router.go: Wrapped c.Request.Body and req.Body with io.LimitReader (10MB) in ParseModelRequest and proxyModelEndpoint.
• pkg/kthena-router/handlers/request.go: Wrapped r.Body with io.LimitReader (10MB) in ParseOpenAIRequestBody.
• pkg/kthena-router/webhook/utils.go: Wrapped r.Body with io.LimitReader (10MB) in parseAdmissionReviewFromRequest.

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[safiya2610]

@LiZhenCheng9527 PTAL.