[3.15] gh-159739: Preserve the syntax error if metadata allocation fails (GH-159751) (#159845)

Restore the original exception when optional syntax-error metadata
cannot be allocated.
(cherry picked from commit 9c239fb)

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

Author: miss-islington

Misc/NEWS.d/next/Core_and_Builtins/2026-10-19-19-33-35.gh-issue-159739.1fc3af.rst

@@ -0,0 +1,2 @@
+Preserve the original :exc:`SyntaxError` if allocating optional error metadata
+fails.

Parser/pegen.c

@@ -926,6 +926,7 @@ _PyPegen_set_syntax_error_metadata(Parser *p) {
     );
     if (!metadata) {
         PyErr_Clear();
+        PyErr_SetRaisedException(exc);
         return;
     }
     PySyntaxErrorObject *syntax_error = (PySyntaxErrorObject *)exc;