Use existing `DEFAULT_FIELDS_SIZE` constant to limit `Account#fields` (#29911)

app/models/account.rb

@@ -64,6 +64,7 @@ class Account < ApplicationRecord
   )
 
   BACKGROUND_REFRESH_INTERVAL = 1.week.freeze
+  DEFAULT_FIELDS_SIZE = 4
   INSTANCE_ACTOR_ID = -99
 
   USERNAME_RE   = /[a-z0-9_]+([a-z0-9_.-]+[a-z0-9_]+)?/i
@@ -103,7 +104,7 @@ class Account < ApplicationRecord
   validates_with UnreservedUsernameValidator, if: -> { local? && will_save_change_to_username? && actor_type != 'Application' }
   validates :display_name, length: { maximum: 30 }, if: -> { local? && will_save_change_to_display_name? }
   validates :note, note_length: { maximum: 500 }, if: -> { local? && will_save_change_to_note? }
-  validates :fields, length: { maximum: 4 }, if: -> { local? && will_save_change_to_fields? }
+  validates :fields, length: { maximum: DEFAULT_FIELDS_SIZE }, if: -> { local? && will_save_change_to_fields? }
   validates :uri, absence: true, if: :local?, on: :create
   validates :inbox_url, absence: true, if: :local?, on: :create
   validates :shared_inbox_url, absence: true, if: :local?, on: :create
@@ -353,8 +354,6 @@ class Account < ApplicationRecord
     self[:fields] = fields
   end
 
-  DEFAULT_FIELDS_SIZE = 4
-
   def build_fields
     return if fields.size >= DEFAULT_FIELDS_SIZE