refactor: use sql_sandbox config instead of env for sync/async

- Replace Application.get_env(:mv, :env) with :sql_sandbox config
- Remove redundant :env config from test.exs
- More explicit and less error-prone for test environment detection

config/test.exs

@@ -47,7 +47,5 @@ config :mv, :session_identifier, :unsafe
 config :mv, :require_token_presence_for_authentication, false
 
 # Enable SQL Sandbox for async LiveView tests
+# This flag controls sync vs async behavior in CycleGenerator after_action hooks
 config :mv, :sql_sandbox, true
-
-# Mark test environment for conditional behavior (e.g., sync vs async operations)
-config :mv, :env, :test