ci: add MinGW build/test job to Windows.yml (#10211)

MinGW silently ignores `#pragma comment(lib, ...)` (MSVC-only), so
linker errors like missing `dbghelp` symbols go undetected until a
downstream user hits them. No CI coverage existed for MinGW on Windows.

## Changes

- **`.github/workflows/Windows.yml`**: New `mingw-build` job using MSYS2
UCRT64 (`mingw-w64-ucrt-x86_64-gcc`) that builds Z3 via `cmake -G Ninja`
and runs `test-z3 /a`, exercising the full link step under MinGW on
every push/PR to master.

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>

src/test/hwf.cpp

@@ -19,6 +19,7 @@ Revision History:
 #include "util/hwf.h"
 #include "util/f2n.h"
 #include "util/rational.h"
+#include <cfloat>
 #include <iostream>
 
 static void bug_set_double() {

src/test/quant_elim.cpp

@@ -497,10 +497,9 @@ void tst_quant_elim() {
    
 
     memory::finalize();
-#ifdef _WINDOWS
+#if defined(_WINDOWS) && defined(_MSC_VER)
     _CrtDumpMemoryLeaks();
 #endif
     exit(0);
 }
 
-