Fix CI: missing include and const-correctness

- Add #include <vector> to type_traits.hpp for the new
  is_compatible_binary_type trait's std::vector<std::uint8_t> reference
  (caught by cpplint's include-what-you-use check)
- Mark test-local json variables const where never reassigned
  (caught by clang-tidy's misc-const-correctness check)

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

tests/src/unit-regression2.cpp

@@ -1140,7 +1140,7 @@ TEST_CASE("regression tests 2")
     {
         // Test that assigning a custom BinaryType directly creates a binary value, not an array
         const std::vector<std::byte> original{std::byte{1}, std::byte{2}, std::byte{3}};
-        json_4804 j = original;
+        const json_4804 j = original;
         CHECK(j.is_binary());
         CHECK(!j.is_array());
 
@@ -1149,7 +1149,7 @@ TEST_CASE("regression tests 2")
         CHECK(extracted == original);
 
         // Test that the default json alias behavior is unchanged: std::vector<uint8_t> -> array
-        json default_json = std::vector<std::uint8_t> {1, 2, 3};
+        const json default_json = std::vector<std::uint8_t> {1, 2, 3};
         CHECK(default_json.is_array());
         CHECK(!default_json.is_binary());
     }
@@ -1158,7 +1158,7 @@ TEST_CASE("regression tests 2")
     {
         // Test that extracting a custom BinaryType from a parsed JSON array still works
         // (not just from a binary-typed node)
-        auto j = json_4804::parse("[1,2,3]");
+        const auto j = json_4804::parse("[1,2,3]");
         CHECK(j.is_array());
         CHECK(!j.is_binary());