telemetry: endpointOf throws on a base path that is an Object prototype key #1652

Found during the v0.12.9 release review (#1647).

endpointOf("GET", "constructor", "/x") throws TypeError: {} is not iterable; so do __proto__ and toString. The route table is looked up without an own-key check. The base path comes only from internal constants (its one caller is api-client.ts), so it cannot be reached with outside input today. Suggested: look up with Object.hasOwn and fall back to the masked form.

Deferred: hardening only, unreachable today.