FIX: Make email_valid handling consistent (#11556)

Previously we were checking truthiness in some places, and `== true` in
others. That can lead to some inconsistent UX where the interface says
the email is valid, but account creation fails.

This commit ensures values are boolean when set, and raises an error for
other value types.

If this safety check is triggered, it means the specific auth provider
needs to be updated to pass booleans.

lib/auth/result.rb

@@ -52,6 +52,13 @@ class Auth::Result
     @email&.downcase
   end
 
+  def email_valid=(val)
+    if !val.in? [true, false, nil]
+      raise ArgumentError, "email_valid should be boolean or nil"
+    end
+    @email_valid = !!val
+  end
+
   def failed?
     !!@failed
   end