Custom BinaryType direct assignment (#5266)

* Fix discussion #4209: custom BinaryType direct assignment and extraction

When a custom BinaryType is configured (other than the default std::vector<uint8_t>),
users can now:
1. Assign values of that type directly to create binary values (not arrays)
2. Extract binary values back to that type with get<>()
3. Extract arrays to that type (for backward compatibility)

Implementation:
- Add is_compatible_binary_type trait to centralize SFINAE condition
- Update to_json to accept custom BinaryType values directly
- Update from_json to handle both binary and array inputs for custom BinaryType
- Add #include <vector> with IWYU comment to from_json.hpp
- Add comprehensive tests for assignment and array extraction
- Update binary_t documentation with example

This is purely additive and invisible to the default nlohmann::json alias, which
continues to treat std::vector<uint8_t> as arrays.

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

* 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>

---------

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

include/nlohmann/detail/conversions/to_json.hpp

@@ -377,6 +377,7 @@ template < typename BasicJsonType, typename CompatibleArrayType,
                          !is_compatible_object_type<BasicJsonType, CompatibleArrayType>::value&&
                          !is_compatible_string_type<BasicJsonType, CompatibleArrayType>::value&&
                          !std::is_same<typename BasicJsonType::binary_t, CompatibleArrayType>::value&&
+                         !is_compatible_binary_type<BasicJsonType, CompatibleArrayType>::value&&
                          !is_basic_json<CompatibleArrayType>::value
 #if JSON_HAS_RANGES && !defined(__MINGW32__)
     && !is_compatible_range_view<CompatibleArrayType>::value
@@ -406,6 +407,14 @@ inline void to_json(BasicJsonType& j, const typename BasicJsonType::binary_t& bi
     external_constructor<value_t::binary>::construct(j, bin);
 }
 
+template < typename BasicJsonType, typename CompatibleArrayType,
+           enable_if_t < is_compatible_binary_type<BasicJsonType, CompatibleArrayType>::value,
+                         int > = 0 >
+inline void to_json(BasicJsonType& j, const CompatibleArrayType& bin)
+{
+    external_constructor<value_t::binary>::construct(j, typename BasicJsonType::binary_t(bin));
+}
+
 template<typename BasicJsonType, typename T,
          enable_if_t<std::is_convertible<T, BasicJsonType>::value, int> = 0>
 inline void to_json(BasicJsonType& j, const std::valarray<T>& arr)