SQL Expressions: Native Support for Dashboard Variables via SQL Functions

### Problem Currently, **dashboard variables in SQL expressions are supported via frontend string interpolation**. While this works, it has some downsides: - The query shown in the UI differs from the actual query sent to the engine. - Escaping and quoting issues can easily arise, especially with strings or lists. ### Proposed Solution Since `go-mysql-server` allows us to define **custom SQL functions**, we should support dashboard variables natively through SQL functions like: `SELECT * FROM my_table WHERE env = dashboard_var('env')` or: `SELECT * FROM my_table WHERE region IN dashboard_var_list('region')` This approach would: - **Avoid escaping bugs** by treating variables as real function inputs. Also Order of Operations issues (when people see something that looks like a var, they tend to thing that changes that OOO, and may forget that the needs to put it in parens.) - **Improve transparency**: the query in the editor is the exact one executed. - **Allow introspection/debugging** of the actual query. ### Benefits - Cleaner and safer query syntax - More consistent user experience - Potentially easier future support for dynamic or cascading variables - Easier testability and repeatability of queries ### TODO / Open Questions - [ ] Define naming conventions for variable functions (`dashboard_var`, `var`, etc.) - [ ] Determine how to expose variable values to the SQL context at runtime - [ ] Decide on behavior when a variable is undefined or empty - [ ] Extend existing SQL test harness to support variable injection