MDEV-18318 Unit tests for the Json_writer

Add 100 TAP unit tests for the Json_writer class covering:

- Invalid JSON detection (unnamed values in objects, named values
  in arrays, mismatched open/close, duplicate keys)
- Output verification for all value types (strings, integers,
  unsigned integers, booleans, null, doubles, sizes)
- Edge cases: LLONG_MAX/MIN, ULLONG_MAX, special doubles (0.0,
  -0.0, HUGE_VAL, NAN), UTF-8 strings, special characters in
  keys and values
- Single-line formatting helper for short arrays
- Nested structures (objects in objects, arrays of objects, mixed)
- RAII wrappers (Json_writer_object, Json_writer_array) including
  NULL writer "ignore mode"
- String_with_limit truncation behavior
- Json_writer size limit enforcement
- add_size formatting (bytes, Kb, Mb)

Signed-off-by: Varun Deep Saini <varun.23bcs10048@ms.sst.scaler.com>

Author: Varun Deep Saini