feat: add Model existsById() helper

[memleakd]

Description

This PR proposes adding existsById() to CodeIgniter\Model.

Sometimes application code only needs to know whether a model row exists for a given primary key. Using find($id) works, but it also fetches and hydrates the row. This helper keeps that intent direct:

$userExists = $userModel->existsById($userId);

It uses the model's configured primary key, respects soft deletes, and can still be combined with query conditions when needed:

$activeUserExists = $userModel->where('active', 1)->existsById($userId);

So this does not introduce a new query path; it is just a small convenience around the existing model existence behavior.

Tests cover existing and missing IDs, soft deletes, constrained model queries, and confirm that this does not trigger find callbacks.

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