Need tests for use of functions in CHECK constraints

We recently enabled the use of deterministic functions in CHECK constraints.

But the current implementation relies on the proper string serialization of function expressions.

But early testing indicates that at least some functions have a string serialization format that isn't round-trip capable, which makes it possible to create tables that break at INSERT time.

We need a test that guarantees that every deterministic function is usable in CHECK constraint.

We also need to tag all dolt-specific functions (DOLT_CHECKOUT etc) as non-deterministic.

Locate the CHECK-constraint handling, function-expression string serialization, and deterministic-function definitions in the repository, then inspect how existing constraint tests are organized. Done means coverage verifies every deterministic function can be used in a CHECK constraint without failing at INSERT time, and Dolt-specific functions such as DOLT_CHECKOUT are tagged non-deterministic.

Written by the indexing model from the issue text.