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.

db/migrate/20250825070005_create_account_saml_settings.rb

@@ -0,0 +1,14 @@
+class CreateAccountSamlSettings < ActiveRecord::Migration[7.1]
+  def change
+    create_table :account_saml_settings do |t|
+      t.references :account, null: false
+      t.string :sso_url
+      t.text :certificate
+      t.string :sp_entity_id
+      t.string :idp_entity_id
+      t.json :role_mappings, default: {}
+
+      t.timestamps
+    end
+  end
+end