gh-159739: Preserve the syntax error if metadata allocation fails

Restore the original exception when optional syntax-error metadata
cannot be allocated.

Backport to 3.15 and 3.14. On 3.14, also remove the erroneous
Py_DECREF(the_source): the N format unit already consumed that
reference.

Author: JelleZijlstra

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

@@ -1083,6 +1083,7 @@ _PyPegen_set_syntax_error_metadata(Parser *p) {
     );
     if (!metadata) {
         PyErr_Clear();
+        PyErr_SetRaisedException(exc);
         return;
     }
     PySyntaxErrorObject *syntax_error = (PySyntaxErrorObject *)exc;