fix: address PR #5788 review comments

- Remove dead `env_vars.get("MODEL")` check in _setup_env (always truthy
  since MODEL is set two lines above)
- Fix test_sync_delegation mock: use return_value instead of side_effect
  list and disable planning to prevent StopAsyncIteration on Python 3.10

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/cli/src/crewai_cli/create_agent.py

@@ -776,7 +776,7 @@ def _setup_env(base: Path, llm_model: str) -> None:
         for details in ENV_VARS.get(provider, [])
         if "key_name" in details
     )
-    if already_set and env_vars.get("MODEL"):
+    if already_set:
         return
 
     if provider in ENV_VARS: