Fix the BON8 CI failures

- compare the float in write_bon8_float with number_float_t constants,
  so GCC does not warn about a float-to-double conversion
- mark check_bon8_utf8's context as used when exceptions are disabled
- choose the compact float prefix in a helper rather than with nested
  conditional operators (clang-tidy)
- use auto for the cast in the BON8 integer reader (clang-tidy)
- write the int32 minimum test values as long long literals (MSVC C4146)

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

include/nlohmann/detail/input/binary_reader.hpp

@@ -3509,7 +3509,7 @@ class binary_reader
         }
 
         const bool negative = second >= 0xC0;
-        std::int64_t value = static_cast<std::int64_t>(negative ? (second & 0x3F) : second);
+        auto value = static_cast<std::int64_t>(negative ? (second & 0x3F) : second);
         std::int64_t offset = 0;
         int extra_bytes = 0;