builtins: ensure that sqlsmith can generate interesting arguments for all builtins

As part of the postportem for #101549, Josh and I discussed how we could have better testing coverage for builtins. One idea mentioned was to rely on `sqlsmith` (which can randomly use all builtins, unless explicitly opted-out). However, we need to make sure that `sqlsmith` can generate "interesting" arguments so that builtin invocation wouldn't result in an error and would actually do something meaningful. E.g. exercising `crdb_internal.probe_ranges` via `sqlsmith` would never get "intersting" arguments because the builtin accepts an enum argument that is never persisted to disk, so it's impossible to actually generate that enum randomly.

We should consider writing a unit test that for each builtin would ensure that "interesting" arguments can picked by `sqlsmith` (unless explicitly opted-out). This would encourage engineers to pick argument types that are more likely to be exercised.

Jira issue: CRDB-27600