Extract buildVisitFields() method in VisitTracker for subclass extensibility

Problem

VisitTracker::trackVisit() builds its insert fields inline, making it impossible for submodules to add extra columns (e.g. domain_id) without duplicating the entire insert logic.

The ShortUrlVisitMiddleware already has a buildVisitFields() method that subclasses can override, but the VisitTracker service does not.

Proposed solution

Extract the field-building logic into a protected buildVisitFields(Redirect $redirect, Request $request) method, matching the pattern already used in the middleware. This allows submodules like domain_shorturl to extend VisitTracker and add extra columns transparently with a single INSERT query.