The new conditionalTest 3-parameter format caused some existing queries to break where a comma was a part of the first parameter value. These are somewhat common if we extend GROUP BY or ORDER BY lists with a conditionalTest macro: Often there is a need to have a leading comma in the string that conditionalTest produces.

Furthermore it would be good IMO to disambiguate if a comma is a delimiter or part of a string properly so there should also be a "proper" form where the commas are escaped.

The 2- vs 3-parameter form of $conditionalTest was decided purely by counting top-level commas, and the comma scan only skipped commas inside parentheses. Any 2-parameter macro whose SQL_if contained a comma was therefore misparsed and the comma dropped, producing broken SQL. Affected cases:

• leading-comma idiom: $conditionalTest(, expr, $var)
• trailing-comma idiom: $conditionalTest(expr,, $var)
• a comma inside a string literal, e.g. AND t = 'a,b', was treated as an
  argument separator and truncated the query.

Fix which maintains the compatibility with the existing style and also provides a better, escaped format:

• Treat a 2-comma macro as 3-parameter only when BOTH branches are non-empty; an empty first/middle segment means the comma belongs to SQL_if.
• Make the comma scan quote-aware ('...', "...", ...) and backslash-aware, so commas inside string literals are never treated as separators.
• Add \, as an explicit escape for a literal comma in SQL_if.
• Document the \, escape in the macro autocomplete tooltip.

Adds regression tests for the leading/trailing-comma idioms, the 3-parameter form, quoted commas (single/double/backtick), escaped quotes, and \, escaping.