test(agent): use python3 in ExecTool workspace scope tests (#5064)

* test(agent): use python3 in ExecTool workspace scope tests (fixes #5062)

* test(agent): use python on Windows and python3 on POSIX in ExecTool workspace scope tests (fixes #5062)

* test(agent): share Python command fixture

---------

Co-authored-by: chengyongru <chengyongru.ai@gmail.com>

tests/agent/conftest.py

@@ -2,6 +2,7 @@
 
 from __future__ import annotations
 
+import os
 from pathlib import Path
 from types import SimpleNamespace
 from unittest.mock import AsyncMock, MagicMock, patch
@@ -13,6 +14,12 @@ from nanobot.bus.queue import MessageBus
 from nanobot.providers.base import LLMProvider
 
 
+@pytest.fixture
+def cmd_python() -> str:
+    """Return the Python command name available to ExecTool tests."""
+    return "python" if os.name == "nt" else "python3"
+
+
 def make_provider(
     default_model: str = "test-model",
     *,