test(windows-shim): stub g_px4_session_id and align errno_map linkage

The runtime shim references g_px4_session_id, an interlocked LONG
defined in platforms/posix/src/px4/windows/runtime/init.cpp. The
test_windows_shim_runtime binary does not link init.cpp because
doing so drags the entire process bootstrap path; without the
symbol the test failed at link time with an unresolved external.

Provide a local definition with C++ linkage that mirrors the
runtime contract (the original is also C++-linkage in init.cpp,
no extern "C"; matching avoids a name-mangling mismatch at link).

Same translation unit had three forward declarations of
errno_map helpers (px4_win_error_to_errno,
px4_wsa_error_to_errno, px4_hstrerror_text) wrapped in
extern "C", but the implementations in errno_map.cpp are C++-
linkage. Drop the extern "C" so the linker resolves to the
mangled names that errno_map.cpp actually exports.

Add the missing <io.h>, <fcntl.h>, and <sys/stat.h> includes
that the runtime tests rely on for _open / _close / _read /
_write declarations under MSVC's CRT.

Signed-off-by: Nuno Marques <n.marques21@hotmail.com>

platforms/posix/src/px4/windows/tests/test_windows_shim_runtime.cpp

@@ -58,7 +58,18 @@
 #include <ws2tcpip.h>
 #include <windows.h>
 
+/* The runtime shim references g_px4_session_id (defined in
+ * platforms/posix/src/px4/windows/runtime/init.cpp). This test binary
+ * does not link init.cpp because pulling it in drags the entire process
+ * bootstrap. Provide a local definition that mirrors the runtime
+ * contract. The original is C++-linkage (no extern "C") in init.cpp so
+ * we match that here exactly. */
+volatile LONG g_px4_session_id = 0;
+
 #include <unistd.h>
+#include <io.h>
+#include <fcntl.h>
+#include <sys/stat.h>
 #include <sys/mman.h>
 #include <sys/file.h>
 #include <pwd.h>
@@ -70,10 +81,12 @@
 
 /* errno_map prototypes -- declared in px4_windows_internal.h but the
  * tests don't include that header (it pulls in the entire winsock+win32
- * stack via the shim umbrella). Forward-declare just what we need. */
+ * stack via the shim umbrella). Forward-declare just what we need. The
-extern "C" int px4_win_error_to_errno(DWORD err);
+ * originals are C++-linkage in errno_map.cpp; matching the linkage
-extern "C" int px4_wsa_error_to_errno(int err);
+ * exactly here keeps name mangling aligned. */
-extern "C" const char *px4_hstrerror_text(int err);
+int px4_win_error_to_errno(DWORD err);
+int px4_wsa_error_to_errno(int err);
+const char *px4_hstrerror_text(int err);
 
 /* errno_map ------------------------------------------------------------ */