feat: add saml model & controller [CW-2958] (#12289)

This PR adds the foundation for account-level SAML SSO configuration in
Chatwoot Enterprise. It introduces a new `AccountSamlSettings` model and
management API that allows accounts to configure their own SAML identity
providers independently, this also includes the certificate generation
flow

The implementation includes a new controller
(`Api::V1::Accounts::SamlSettingsController`) that provides CRUD
operations for SAML configuration

The feature is properly gated behind the 'saml' feature flag and
includes administrator-only authorization via Pundit policies.

config/routes.rb

@@ -69,6 +69,7 @@ Rails.application.routes.draw do
             end
             resources :documents, only: [:index, :show, :create, :destroy]
           end
+          resource :saml_settings, only: [:show, :create, :update, :destroy]
           resources :agent_bots, only: [:index, :create, :show, :update, :destroy] do
             delete :avatar, on: :member
             post :reset_access_token, on: :member