Replace variation key use with SHA256 of key to prevent long filenames

If a variant has a large set of options associated with it, the generated
filename will be too long, causing Errno::ENAMETOOLONG to be raised. This
change replaces those potentially long filenames with a much more compact
SHA256 hash. Fixes #30662.

activestorage/app/models/active_storage/variant.rb

@@ -50,7 +50,7 @@ def processed
 
   # Returns a combination key of the blob and the variation that together identifies a specific variant.
   def key
-    "variants/#{blob.key}/#{variation.key}"
+    "variants/#{blob.key}/#{Digest::SHA256.hexdigest(variation.key)}"
   end
 
   # Returns the URL of the variant on the service. This URL is intended to be short-lived for security and not used directly

activestorage/test/models/variant_test.rb

@@ -26,4 +26,9 @@ class ActiveStorage::VariantTest < ActiveSupport::TestCase
     assert_equal 67, image.height
     assert_match(/Gray/, image.colorspace)
   end
+
+  test "service_url doesn't grow in length despite long variant options" do
+    variant = @blob.variant(font: "a" * 10_000).processed
+    assert_operator variant.service_url.length, :<, 500
+  end
 end