AP-5141: Add test to validate document length

If a new file name is added that is too long, e.g.

```ruby
  VALID_DOCUMENT_TYPES = %w[
    part_bank_state_evidence_pdf
    this_file_name_is_far_too_long_pdf
  ].freeze
```

Then the test will fail with

```ruby
Failures:

  1) DocumentCategoryValidator VALID_DOCUMENT_TYPE length has 30 characters or fewer to be valid
     Failure/Error: expect(doc_types).to be_empty

                            Expected { "this_file_name_is_far_too_long_pdf" => 34 }
       to return a truthy result for empty? or emptys?
```

So it will show the offending file name and how. long it is

spec/validators/document_category_validator_spec.rb

@@ -7,6 +7,15 @@ class DummyDocumentCategory
 end
 
 RSpec.describe DocumentCategoryValidator do
+  describe "VALID_DOCUMENT_TYPE length" do
+    it "has 30 characters or fewer to be valid" do
+      doc_types = DocumentCategoryValidator::VALID_DOCUMENT_TYPES
+                    .index_with(&:length)
+                    .reject { |_k, v| v <= 30 }
+      expect(doc_types).to be_empty
+    end
+  end
+
   context "with Attachment" do
     subject(:document_category_validator) { Attachment.create! legal_aid_application: laa, attachment_type: }