chore(api): remove the unused public API route builder and dead endpoint labels

`withPublicApiRouteHandler` and 27 `ApiEndpoint` union members landed together
in #5273, but the v2 surface shipped on `defineV2JsonRoute` + `v2RateLimits`
instead. The builder had no production caller — only its own test — and the v2
rate limiter never reads an `ApiEndpoint` label, so those members were never
emitted to telemetry by symbol or by string literal.

Remaining members are exactly the labels a v1 route passes to `checkRateLimit`
or `authenticateRequest`. Drops the now-unreachable `hasZodUsage` branch from
the API validation audit; no ratchet metric moves (route total stays 1093).

Author: waleedlatif1