cbc: add FileNotFoundError for missing libCbc.dll on Windows

The cbcbox Windows wheel currently only ships libCbc.a (static archive)
with no shared libCbc.dll. Add an explicit check so the error message
clearly identifies the missing file instead of a cryptic dlopen failure.

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

Author: h-g-s

mip/cbc.py

@@ -83,6 +83,12 @@
             # autotools/MinGW places DLLs under bin/, not lib/
             _bin_dir = os.path.join(_cbcbox.cbc_dist_dir(), "bin")
             libfile = os.path.join(_bin_dir, "libCbc.dll")
+            if not os.path.exists(libfile):
+                raise FileNotFoundError(
+                    "libCbc.dll not found in cbcbox Windows distribution at"
+                    " {}. The cbcbox Windows wheel may only contain a static"
+                    " libCbc.a. A shared libCbc.dll is required.".format(_bin_dir)
+                )
             # Python 3.8+ ignores PATH for DLL resolution; use add_dll_directory
             if hasattr(os, "add_dll_directory"):
                 os.add_dll_directory(_bin_dir)