Fix CI: use VS 2026 on windows-11-arm and use raw string literals in tests (#5575)

The windows-11-arm runner image moved to windows-11-vs2026-arm64, which no
longer ships Visual Studio 2022, so the msvc-arm64 job failed at configure
time. Use the "Visual Studio 18 2026" generator like the msvc2026 job.

clang-tidy's modernize-raw-string-literal check flagged two string literals
in the nesting tests added by #5546 and #5547.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

tests/src/unit-merge_patch.cpp

@@ -62,7 +62,7 @@ std::string nested_objects(const std::size_t depth, const int variant)
         }
         text += "\"a\":";
     }
-    text += variant == 1 ? "{\"x\":1,\"y\":null}" : "{\"y\":2}";
+    text += variant == 1 ? R"({"x":1,"y":null})" : "{\"y\":2}";
     text.append(depth, '}');
     return text;
 }

tests/src/unit-modifiers.cpp

@@ -48,7 +48,7 @@ std::string nested_objects(const std::size_t depth, const int variant)
         if (variant == 2 && i % 5 == 0)
         {
             // an object replacing a primitive, which is not merged
-            text += "\"s0\":{\"o\":1},";
+            text += R"("s0":{"o":1},)";
         }
         text += "\"a\":";
     }