Issue a short-lived SQL warehouse credential to the caller's kernel.

A running kernel cannot receive an updated environment, so a token injected at kernel start would be pinned to a value that expires within the hour. The kernel calls this per connection instead; nothing durable is written into its environment.

Runtime-only, because a kernel or a sandbox script holds an internal principal token and no cookie. The session and API-key dependency the rest of this router uses rejects that token, so this route would answer nothing that ever calls it. No browser surface needs a Databricks token either.