Test all three custom host headers

src/native/corehost/coreclr_delegates.h

@@ -1,11 +1,17 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
-#ifndef __CORECLR_DELEGATES_H__
-#define __CORECLR_DELEGATES_H__
+#ifndef HAVE_CORECLR_DELEGATES_H
+#define HAVE_CORECLR_DELEGATES_H
 
+#include <stddef.h>
 #include <stdint.h>
 
+#ifdef __cplusplus
+extern "C"
+{
+#endif
+
 #if defined(_WIN32)
     #define CORECLR_DELEGATE_CALLTYPE __stdcall
     #ifdef _WCHAR_T_DEFINED
@@ -57,4 +63,8 @@ typedef int (CORECLR_DELEGATE_CALLTYPE *load_assembly_bytes_fn)(
     void       *load_context        /* Extensibility parameter (currently unused and must be 0) */,
     void       *reserved            /* Extensibility parameter (currently unused and must be 0) */);
 
-#endif // __CORECLR_DELEGATES_H__
+#ifdef __cplusplus
+}
+#endif // __cplusplus
+
+#endif // HAVE_CORECLR_DELEGATES_H

src/native/corehost/nethost/nethost.h

@@ -1,8 +1,8 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
-#ifndef __NETHOST_H__
-#define __NETHOST_H__
+#ifndef HAVE_NETHOST_H
+#define HAVE_NETHOST_H
 
 #include <stddef.h>
 
@@ -37,7 +37,8 @@
 #endif
 
 #ifdef __cplusplus
-extern "C" {
+extern "C"
+{
 #endif
 
 // Parameters for get_hostfxr_path
@@ -93,7 +94,7 @@ NETHOST_API int NETHOST_CALLTYPE get_hostfxr_path(
     const struct get_hostfxr_parameters *parameters);
 
 #ifdef __cplusplus
-} // extern "C"
-#endif
+}
+#endif // __cplusplus
 
-#endif // __NETHOST_H__
+#endif // HAVE_NETHOST_H

src/native/corehost/test/mockhostfxr/test_c_api.c

@@ -1,8 +1,10 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
-// hostfxr.h is a public API. When included in .c files, it may fail to compile
-// if C++-specific syntax is used within the extern "C" block. Since all usage of
-// this API in runtime repo is within C++ code, such breakages are not encountered
-// during normal development or testing.
-#include "hostfxr.h"
+// These headers provide access to the public hosting APIs. This file tests that
+// they can be included in pure C code without causing compilation errors.
+// Since the runtime repository primarily uses these APIs in C++ code, such issues
+// might not be caught during regular development or testing.
+#include <coreclr_delegates.h>
+#include <hostfxr.h>
+#include <nethost/nethost.h>