feat(cloudsql-instance): add retain_backups_on_delete and final_backup_config support

- Add retain_backups_on_delete setting to allow backups to be retained when instance is deleted
- Add final_backup_config with enabled flag and retention_days configuration
- Both settings default to disabled/null for backward compatibility
- Prevents accidental loss of backups through Terraform operations

modules/cloudsql-instance/main.tf

@@ -61,6 +61,7 @@ resource "google_sql_database_instance" "primary" {
     collation                   = var.collation
     connector_enforcement       = var.connector_enforcement
     time_zone                   = var.time_zone
+    retain_backups_on_delete    = var.backup_configuration.retain_backups_on_delete
 
     ip_configuration {
       ipv4_enabled = var.network_config.connectivity.public_ipv4
@@ -127,6 +128,14 @@ resource "google_sql_database_instance" "primary" {
       }
     }
 
+    dynamic "final_backup_config" {
+      for_each = var.backup_configuration.final_backup != null ? [var.backup_configuration.final_backup] : []
+      content {
+        enabled        = final_backup_config.value.enabled
+        retention_days = final_backup_config.value.retention_days
+      }
+    }
+
     dynamic "connection_pool_config" {
       # Only available for PostgreSQL on Enterprise Plus edition.
       for_each = (var.managed_connection_pooling_config.enabled && local.is_postgres && var.edition == "ENTERPRISE_PLUS") ? [1] : []