perf: reuse eager-loaded relations in forModel via loadMissing

[MACscr]

What

BaseBuilder::forModel() unconditionally calls $model->load('customFieldValues.customField.options') on every Infolist/Form schema generation. When the host application already eager-loads those relations — e.g. a Filament resource's getEloquentQuery() loading custom field values so the table avoids an N+1 — load() refetches them anyway, so a view/edit page runs the three relation queries twice: once from the host's eager-load, once from forModel().

This switches load() → loadMissing(), so forModel() reuses relations that are already loaded and only queries when they're absent.

Why it's safe

• The un-eager path is unchanged: loadMissing() still loads the relations when they aren't present (covered by a test).
• It matches the package's own behaviour elsewhere — BackendVisibilityService::extractFieldValues() and LookupPreloader already guard on relationLoaded() rather than force-loading.
• The TableBuilder guard is untouched; only Infolist/Form/Exporter/Importer builders are affected, and Exporter/Importer call forModel(new $model) on a keyless instance where the load is inert.

Caveat

loadMissing() no longer force-refetches. If code mutates a record's custom field values and re-renders the same in-memory instance within one request, it should $record->unsetRelation('customFieldValues') before re-rendering. Standard Filament view/edit flows re-resolve the record per request, so they're unaffected.

Test

tests/Feature/Integration/Builders/ForModelLoadMissingTest.php:

• asserts forModel() issues no custom_field_values query when the record is already eager-loaded (fails on load(), passes on loadMissing())
• asserts the relations still load when the record isn't eager-loaded (regression guard)

Local run of the full CI gate passes: Pint, PHPStan, Rector, and Pest (785 passed).