DEV: Fix various rubocop lints (#24749)

These (21 + 3 from previous PRs) are soon to be enabled in rubocop-discourse:

Capybara/VisibilityMatcher
Lint/DeprecatedOpenSSLConstant
Lint/DisjunctiveAssignmentInConstructor
Lint/EmptyConditionalBody
Lint/EmptyEnsure
Lint/LiteralInInterpolation
Lint/NonLocalExitFromIterator
Lint/ParenthesesAsGroupedExpression
Lint/RedundantCopDisableDirective
Lint/RedundantRequireStatement
Lint/RedundantSafeNavigation
Lint/RedundantStringCoercion
Lint/RedundantWithIndex
Lint/RedundantWithObject
Lint/SafeNavigationChain
Lint/SafeNavigationConsistency
Lint/SelfAssignment
Lint/UnreachableCode
Lint/UselessMethodDefinition
Lint/Void

Previous PRs:
Lint/ShadowedArgument
Lint/DuplicateMethods
Lint/BooleanSymbol
RSpec/SpecFilePathSuffix

plugins/chat/app/services/chat/update_message.rb

@@ -111,7 +111,7 @@ module Chat
 
     def should_create_revision(new_message, prev_message, guardian)
       max_seconds = SiteSetting.chat_editing_grace_period
-      seconds_since_created = Time.now.to_i - new_message&.created_at.iso8601.to_time.to_i
+      seconds_since_created = Time.now.to_i - new_message&.created_at&.iso8601&.to_time.to_i
       return true if seconds_since_created > max_seconds
 
       max_edited_chars =

plugins/chat/app/services/service/base.rb

@@ -38,7 +38,7 @@ module Service
       #   context.fail!("failure": "something went wrong")
       # @return [Context]
       def fail!(context = {})
-        fail(context)
+        self.fail(context)
         raise Failure, self
       end