rpcserver: add WithOrigin for middleware integration

[odysseus0]

Summary

• Adds WithOrigin(r *http.Request, origin string) *http.Request
• Symmetric with existing GetOrigin(ctx) - completes the API

Motivation

Middleware that validates API keys and derives origin programmatically needs a way to set origin in the request context. Previously, the only option was to set an HTTP header and rely on ExtractOriginFromHeader, which conflates internal context passing with external headers.

Usage

func authMiddleware(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        origin := validateAPIKey(r.Header.Get("X-API-Key"))
        r = rpcserver.WithOrigin(r, origin)
        next.ServeHTTP(w, r)
    })
}

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