Add comprehensive test coverage for EXISTS/NOT EXISTS subquery functionality

[Copilot]

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.

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• 360.zef.pm
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• esm.ubuntu.com
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If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

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