test: cover has-one-through initial constraints

[elpete]

Closes #119

Review

This bug report is a strong fit for Quick. A hasOneThrough chain should support any valid relationship as its first hop, including hasOne; otherwise composition depends on an internal subclass detail.

Recommendation: 10/10.

Reasons for

• Relationship chains should behave consistently across hasOne and hasMany first hops.
• The failure was an internal method-placement bug, not an unsupported relationship shape.
• The exact scenario is compact to preserve through the public API.

Reasons against

• Deep relationship SQL is inherently more complex, so every additional relationship combination expands the regression surface.

Attempted reproduction

I modeled the same relationship shape through the public API: User.favoritePost is a hasOne, and Post.author is a belongsTo; User.favoritePostAuthor composes them with hasOneThrough.

The historical initialThroughConstraints missing-method error no longer reproduces on current next with qb@14.0.0-beta.3. Commit 7b3b8d0 moved initialThroughConstraints() from HasMany to the shared HasOneOrMany implementation. This PR adds direct public-API coverage for that fix.

Validation

• Focused has-one-through spec: 2 passed, 0 failed, 0 errors
• Full suite: 496 passed, 0 failed, 0 errors, 3 skipped
• Formatter completed
• git diff --check passed