Fix 'make dev' failure in Windows environment (#3236)

* fix: Solving the problem of "make dev" failing to start in Windows environment

* fix: revert the change to the startup_config and fix the lint errors

* fix: Address Copilot review feedback

- Validate wait-for-port input and avoid PowerShell port interpolation
- Require Python 3 in serve.sh launcher detection
- Keep Windows event loop policy setup in sitecustomize only
- Clarify sitecustomize process-wide backend behavior

backend/sitecustomize.py

@@ -0,0 +1,26 @@
+"""Process-wide Python startup customizations for backend entrypoints.
+
+When ``backend/`` is on ``sys.path``, Python imports this module during
+interpreter startup. Keep changes here suitable for all gateway, script,
+migration, and test entrypoints that run in that environment.
+"""
+
+from __future__ import annotations
+
+import asyncio
+import sys
+
+
+def _configure_windows_event_loop_policy() -> None:
+    if sys.platform != "win32":
+        return
+
+    selector_policy = getattr(asyncio, "WindowsSelectorEventLoopPolicy", None)
+    if selector_policy is None:
+        return
+
+    if not isinstance(asyncio.get_event_loop_policy(), selector_policy):
+        asyncio.set_event_loop_policy(selector_policy())
+
+
+_configure_windows_event_loop_policy()