fix(proxy): override Vercel build command so it doesn't run turbo (#13448)

The proxy is intentionally outside the pnpm workspace and has no build
step (it ships as Vercel Functions compiled from api/). Vercel was
auto-detecting Turbo and defaulting the build command to `turbo run
build`, which fails with "Could not find task build in project" because
the isolated proxy package has no turbo.json or build task.

Add a vercel.json with an explicit no-op buildCommand so Vercel skips
turbo; the api/ functions are still detected and deployed.