cbc: fix isfile import and Cbc_reset before re-solve

Two fixes for newer CBC (cbcbox 2.901):

1. Add missing 'isfile' import from os.path in cbc.py (used in
   SolverCbc.read() but was not imported, causing NameError in
   mip_files_test).

2. Call Cbc_reset() before Cbc_solve() to clear stale solution state
   when re-solving the same model. Newer CBC's Cbc_solve() returns
   the old objective value without a reset. Cbc_reset() also resets
   the objective sense to MINIMIZE, so save and restore it around
   the call.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: h-g-s

mip/cbc.py

@@ -3,7 +3,7 @@
 import logging
 from typing import Dict, List, Tuple, Optional, Union
 from sys import platform, maxsize
-from os.path import dirname
+from os.path import dirname, isfile
 import os
 import multiprocessing as multip
 import numbers
@@ -1222,6 +1222,12 @@ def cbc_cut_callback(osi_solver, osi_cuts, app_data, depth, npass):
         )
 
         self.__clear_sol()
+        # Cbc_reset clears previous solve state (required in newer CBC to
+        # avoid stale results when re-solving), but also resets objective
+        # sense, so we save and restore it.
+        _sense = cbclib.Cbc_getObjSense(self._model)
+        Cbc_reset(self._model)
+        cbclib.Cbc_setObjSense(self._model, _sense)
         cbclib.Cbc_solve(self._model)
 
         if cbclib.Cbc_isAbandoned(self._model):