Add OIDC fixes, add prefered_username, add application-user claims

app/models/group.rb

@@ -4,11 +4,31 @@ class Group < ApplicationRecord
   has_many :application_groups, dependent: :destroy
   has_many :applications, through: :application_groups
 
+  # Reserved OIDC claim names that should not be overridden
+  RESERVED_CLAIMS = %w[
+    iss sub aud exp iat nbf jti nonce azp
+    email email_verified preferred_username name
+    groups
+  ].freeze
+
   validates :name, presence: true, uniqueness: { case_sensitive: false }
   normalizes :name, with: ->(name) { name.strip.downcase }
+  validate :no_reserved_claim_names
 
   # Parse custom_claims JSON field
   def parsed_custom_claims
-    custom_claims || {}
+    return {} if custom_claims.blank?
+    custom_claims.is_a?(Hash) ? custom_claims : {}
+  end
+
+  private
+
+  def no_reserved_claim_names
+    return if custom_claims.blank?
+
+    reserved_used = parsed_custom_claims.keys.map(&:to_s) & RESERVED_CLAIMS
+    if reserved_used.any?
+      errors.add(:custom_claims, "cannot override reserved OIDC claims: #{reserved_used.join(', ')}")
+    end
   end
 end