test: compare whereHas through constraints

[elpete]

Closes #116

Review

This is a core correctness issue for Quick. A composed hasManyThrough relationship must apply the same intermediate and callback constraints as the equivalent expanded relationship path.

Recommendation: 10/10.

Reasons for

• whereHas is commonly used for authorization and filtering, so over-inclusive results are dangerous.
• A named through relationship should be semantically equivalent to its expanded path.
• The regression can be asserted through the public query API with deterministic fixture data.

Reasons against

• Through relationship SQL is complex and fixes can affect eager loading, nested constraints, and aliases, so broad regression testing is necessary.

Attempted reproduction

I compared Country.whereHas( "posts", callback ), where posts is hasManyThrough([ "users", "posts" ]), with Country.whereHas( "users.posts", callback ). Both callbacks constrain the terminal post to post_pk = 321.

The issue no longer reproduces on current next with qb@14.0.0-beta.3: both forms return only Argentina. The relevant historical fixes include correcting through-constraint application and removing an extra join from has/whereHas that pulled back too many rows. This PR preserves exact equivalence coverage.

Validation

• Focused querying-relationships spec: 38 passed, 0 failed, 0 errors
• Full suite: 496 passed, 0 failed, 0 errors, 3 skipped
• Formatter completed
• git diff --check passed