[12.x] Refactor Eloquent Builder: Add new addWhereRelationCallback method to reduce code duplication

[ezequidias]

This PR introduces a new internal method called addWhereRelationCallback, designed to remove duplicated logic found across several relationship-based query methods in Eloquent.

Previously, the following methods contained nearly identical closure logic to apply conditions on related models:

• withWhereRelation
• whereRelation
• orWhereRelation
• whereDoesntHaveRelation
• orWhereDoesntHaveRelation

Each of these methods used an inline closure to check if the $column parameter was a Closure or a column name, then applied the condition accordingly.

By introducing addWhereRelationCallback, we now reuse a single method that handles this logic, resulting in:

• Cleaner and more maintainable code
• Less duplication
• More consistent behavior across methods

No behavior has been changed—this is a pure refactor aimed at improving code quality.

[taylorotwell]

I think it's fine for now.