Reload nested cart relations after refresh (#2222)

[wakqasahmed]

Closes #2222.

The reporter's own diagnosis in the issue was spot on: Cart::add(), remove(), updateLine() and the other cart mutators all call $this->refresh() before recalculating, and Eloquent's refresh() only reloads relations that are already loaded, one level deep. So after the first add, lines.purchasable.prices (and the rest of the nested lunar.cart.eager_load config) gets dropped down to just lines, and the next calculation lazy-loads it. With lazy loading disabled that's the LazyLoadingException from adding a second product.

I added Cart::refreshForCalculation(), which refreshes the cart and then reapplies the full eager_load config, and swapped it in everywhere the old refresh()->recalculate() pattern was used. Made it public since it's a reasonable thing for anyone building custom cart flows to reuse.

I built on the failing test from #2221 (closed) rather than duplicating that diagnostic work — added a regression test that asserts the nested relations survive a refresh, plus the two-different-products-in-one-request scenario from that PR.

One thing worth flagging: I deliberately kept this scoped to the refresh mechanism the issue actually describes. While testing with Model::preventLazyLoading() turned on globally, adding a second product still eventually hits separate, unrelated lazy loads a few frames deeper in the pricing/tax calculation (ProductVariant::taxClass, then TaxClass::taxRateAmounts, then further into discounts). That matches what came up in #2221's thread - alecritson noted supporting preventLazyLoading app-wide would need a rethink of how pricing works, and closed that PR rather than patch around it. This PR doesn't attempt that; it just fixes the specific refresh bug reported here.

Ran ./vendor/bin/pint and the full core test suite locally (Pest, sqlite) - all cart tests pass, 584 passed overall, the only failures are pre-existing environment gaps unrelated to this change (missing GD extension in my local PHP build, hit by MediaObserverTest and HasMediaTraitTest).