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/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema[7.1].define(version: 2025_08_22_061042) do
+ActiveRecord::Schema[7.1].define(version: 2025_08_25_070005) do
   # These extensions should be enabled to support this database
   enable_extension "pg_stat_statements"
   enable_extension "pg_trgm"
@@ -28,6 +28,18 @@ ActiveRecord::Schema[7.1].define(version: 2025_08_22_061042) do
     t.index ["token"], name: "index_access_tokens_on_token", unique: true
   end
 
+  create_table "account_saml_settings", force: :cascade do |t|
+    t.bigint "account_id", 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.datetime "created_at", null: false
+    t.datetime "updated_at", null: false
+    t.index ["account_id"], name: "index_account_saml_settings_on_account_id"
+  end
+
   create_table "account_users", force: :cascade do |t|
     t.bigint "account_id"
     t.bigint "user_id"