fix: downcase email before finding (#8921)

* fix: downcase email when finding

* feat: add `from_email` class

* refactor: use `from_email`

* feat: add rule to disallow find_by email directly

* chore:  remove redundant test

Since the previous imlpmentation didn't do a case-insentive search, a new user would be created, and the error would be raised at the DB layer. With the new changes, this test case is redundant

* refactor: use from_email

lib/rubocop/user_find_by.rb

@@ -0,0 +1,16 @@
+require 'rubocop'
+
+# Enforces use of from_email for email attribute lookups
+class UseFromEmail < RuboCop::Cop::Cop
+  MSG = 'Use `from_email` for email lookups to ensure case insensitivity.'.freeze
+
+  def_node_matcher :find_by_email?, <<~PATTERN
+    (send _ :find_by (hash (pair (sym :email) _)))
+  PATTERN
+
+  def on_send(node)
+    return unless find_by_email?(node)
+
+    add_offense(node, message: MSG)
+  end
+end