[3.15] gh-159739: Skip context watchers cleared during notification (GH-160164) (#160291)

A context watcher callback can clear a later watcher while a context-switch notification is being dispatched. The dispatcher still uses its earlier snapshot of active watchers, so it calls a NULL callback and crashes; check that each callback is still registered before invoking it. Backport to 3.15 and 3.14.
(cherry picked from commit 0a57626)

Co-authored-by: Jelle Zijlstra <jelle@openai.com>

Author: miss-islington

Misc/NEWS.d/next/C_API/2026-10-21-19-55-00.gh-issue-159739.ctxt01.rst

@@ -0,0 +1,2 @@
+Avoid a crash if a context watcher is cleared while other context watchers
+are being notified.

Python/context.c

@@ -169,8 +169,8 @@ notify_context_watchers(PyThreadState *ts, PyContextEvent event, PyObject *ctx)
         assert(i < CONTEXT_MAX_WATCHERS);
         if (bits & 1) {
             PyContext_WatchCallback cb = interp->context_watchers[i];
-            assert(cb != NULL);
-            if (cb(event, ctx) < 0) {
+            // A previous callback may have cleared this watcher.
+            if (cb != NULL && cb(event, ctx) < 0) {
                 PyErr_FormatUnraisable(
                     "Exception ignored in %s watcher callback for %R",
                     context_event_name(event), ctx);