feat: add Query Builder exists and doesntExist methods

[memleakd]

Description

This PR proposes adding exists() and doesntExist() to Query Builder.

These methods make the common "does anything match this builder?" check simple without forcing users to fetch rows or write a count query manually:

$builder->where('status', 'pending');

if ($builder->exists()) {
    // At least one pending row exists.
}

The implementation compiles a lightweight existence probe, preserves the builder state when $reset is false, and follows the existing Query Builder test-mode behavior by returning SQL instead of executing it.

It also handles less obvious builder shapes like limit() / offset(), grouped queries, having(), unions, reset behavior, and failed query execution. If the existence probe cannot be executed, both methods return false.

This is separate from whereExists(): exists() executes the current builder to check whether matching rows exist, while whereExists() adds an SQL EXISTS predicate to another query.

Changes

• Added BaseBuilder::exists() and BaseBuilder::doesntExist().
• Added Model PHPDoc entries for the new builder methods.
• Added user guide docs, class reference entries, and changelog entry.
• Added focused builder and live database tests.

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value (without duplication)
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide