electric-feat-add-variadic-function

## Task

At parse time, the sync services are rejecting shapes whose WHERE clauses use `coalesce(...)`, `greatest(...)`, or `least(...)`. We need to add support for these in the eval engine. Should treat these the way Postgres does: accepting any number of arguments of compatible types, ignoring nulls (or returning first non-null for coalesce), and short-circuiting when an earlier argument settles the result.

## User stories / requirements

- The WHERE-clause expression engine supports `coalesce(...)`, which accepts any number of arguments and returns the first non-null argument. When every argument is null, it should return null. `coalesce(...)` should short-circuit and return the first non-null argument.
- The expression engine supports `greatest(...)` and `least(...)` with an arbitrary number of arguments. Both `greatest(...)` and `least(...)` skip nulls and return the largest or smallest non-null argument respectively.
- If someone writes a caller passing an array via `func(VARIADIC ARRAY[...])` Postgres syntax, they should get a clear error that mentions VARIADIC.

## General instructions

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