Fix Python 3.13 GIL and event loop compatibility

- Add gil_acquire()/gil_release() helpers that check PyGILState_Check()
  before calling PyGILState_Ensure() to avoid double-acquisition on Python 3.12+
- Fix event loop thread isolation: ErlangEventLoop for main thread only,
  SelectorEventLoop for worker threads
- Worker threads in py_callback.c now create SelectorEventLoop directly,
  bypassing the policy to avoid ErlangEventLoop conflicts
- Use per-call ErlNifEnv for timer scheduling to prevent races in
  free-threaded Python mode
- Fail fast on RuntimeError in erlang_loop.py instead of silently
  swallowing initialization errors that cause hangs

Fixes py_async_e2e_SUITE timeouts on Python 3.12+ by ensuring proper
thread isolation for asyncio event loops.

Author: benoitc

c_src/py_callback.c

@@ -1654,8 +1654,21 @@ static void *async_event_loop_thread(void *arg) {
         return NULL;
     }
 
-    /* Create new event loop */
-    PyObject *loop = PyObject_CallMethod(asyncio, "new_event_loop", NULL);
+    /* Create a default selector event loop directly, bypassing the policy.
+     * Worker threads should NOT use ErlangEventLoop since it requires the
+     * main thread's event router. Using SelectorEventLoop ensures these
+     * background threads have their own independent event loops. */
+    PyObject *selector_loop_class = PyObject_GetAttrString(asyncio, "SelectorEventLoop");
+    PyObject *loop = NULL;
+    if (selector_loop_class != NULL) {
+        loop = PyObject_CallObject(selector_loop_class, NULL);
+        Py_DECREF(selector_loop_class);
+    }
+    if (loop == NULL) {
+        /* Fallback to new_event_loop if SelectorEventLoop not available */
+        PyErr_Clear();
+        loop = PyObject_CallMethod(asyncio, "new_event_loop", NULL);
+    }
     if (loop == NULL) {
         PyErr_Print();
         Py_DECREF(asyncio);
@@ -1664,7 +1677,7 @@ static void *async_event_loop_thread(void *arg) {
         return NULL;
     }
 
-    /* Set as current loop */
+    /* Set as current loop for this thread */
     PyObject *set_result = PyObject_CallMethod(asyncio, "set_event_loop", "O", loop);
     Py_XDECREF(set_result);