Stop dropping media custom properties on create/edit

[wakqasahmed]

Closes #2596

Summary

MediaRelationManager treated custom_properties as a closed set of exactly two keys (name, primary), which silently broke Spatie Media Library's open key/value contract in two ways:

1. Create — the CreateAction handler enumerated only name/primary from the submitted form data and discarded everything else, so any field added via the documented RelationManagerExtension::extendForm() seam rendered, validated, and submitted — but its value was never persisted.
2. Edit — EditAction wrote the submitted form data straight onto the model. Because custom_properties is an array-cast column, this replaced the whole JSON value instead of merging into it, so any key not rendered by the edit form (imported metadata, dedup markers, alt text, etc.) was silently dropped on save.

Fix

• Create: pass the full submitted custom_properties array through to withCustomProperties() instead of hand-picking name/primary.
• Edit: added mutateDataUsing() to the EditAction that merges the submitted custom_properties on top of the record's existing custom_properties, so unrendered keys survive while submitted keys still win.

Testing

• Added a feature test asserting a custom property contributed via a RelationManagerExtension::extendForm() field is persisted when a media item is created.
• Added a feature test asserting editing a media item preserves an existing custom property key that the edit form does not render, while still applying the change to the field the form does render.
• Ran vendor/bin/pest tests/admin/Feature/Support (built a PHP 8.3 + intl/exif/bcmath/gd toolchain via Docker to install the monorepo's Composer dependencies): all 16 tests in that directory pass, including the 2 new tests and the pre-existing PriceRelationManagerTest/RelationManagerExtensionTest/CreatePageExtensionTest suites.
• Verified the new "persists custom properties added by an extension when creating media" test fails against the pre-fix code (null instead of 'Jane Doe'), confirming it actually exercises bug 1.
• php -l on both changed files (no syntax errors).