This PR adds comprehensive test coverage for the EXISTS/NOT EXISTS subquery functionality that was already implemented in Red ORM but not thoroughly tested.

Background

Issue #578 requested the ability to use EXISTS/NOT EXISTS as subqueries in Red ORM. The functionality was already implemented via the exists, is-empty, and manual Red::AST::Function approaches, but lacked proper test coverage to ensure it works as expected.

Changes

• Added t/89-exists-subquery.rakutest with comprehensive test coverage for:
  • .exists method generating correct SQL with EXISTS clauses
  • .exists.not method generating correct SQL with NOT EXISTS clauses
  • .is-empty method (equivalent to .exists.not)
  • Manual Red::AST::Function approach with NOT EXISTS
  • Complex queries combining EXISTS checks with other conditions

Functionality Confirmed

All approaches mentioned in the original issue now work correctly:

# Recommended approach
my $last = StatsViewRefresh.^rs.max(*.refresh-time);
my $exists-not = StatsViewRefresh.^rs.exists.not;
Upload.^all.grep( -> $v { $exists-not OR $v.uploaded > $last }).so;

# Alternative approaches also work
StatsViewRefresh.^rs.is-empty
Red::AST::Function.new(args => [StatsViewRefresh.^rs.map(*.id).ast], func => 'NOT EXISTS')

These generate proper SQL with EXISTS/NOT EXISTS clauses as requested.

Fixes #578.

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