[3.12] pythongh-130077: Properly match full soft keywords in the parser (pythonGH-135317)

(cherry picked from commit ff2b5f4)

Co-authored-by: Pablo Galindo Salgado <Pablogsal@gmail.com>

Author: pablogsal

Lib/test/test_syntax.py

@@ -322,6 +322,13 @@
 Traceback (most recent call last):
 SyntaxError: invalid syntax
 
+# But prefixes of soft keywords should
+# still raise specialized errors
+
+>>> (mat x)
+Traceback (most recent call last):
+SyntaxError: invalid syntax. Perhaps you forgot a comma?
+
 From compiler_complex_args():
 
 >>> def f(None=1):

Misc/NEWS.d/next/Core and Builtins/2025-06-09-23-57-37.gh-issue-130077.MHknDB.rst

@@ -0,0 +1,2 @@
+Properly raise custom syntax errors when incorrect syntax containing names
+that are prefixes of soft keywords is encountered.  Patch by Pablo Galindo.

Parser/pegen.c

@@ -637,7 +637,8 @@ expr_ty _PyPegen_soft_keyword_token(Parser *p) {
     Py_ssize_t size;
     PyBytes_AsStringAndSize(t->bytes, &the_token, &size);
     for (char **keyword = p->soft_keywords; *keyword != NULL; keyword++) {
-        if (strncmp(*keyword, the_token, size) == 0) {
+        if (strlen(*keyword) == (size_t)size &&
+            strncmp(*keyword, the_token, (size_t)size) == 0) {
             return _PyPegen_name_from_token(p, t);
         }
     }