fix #13185 & #13192: move inline bool operator to json.h

[maloel]

PR #13192 caused warnings under Linux:

In file included from /home/jenkins/workspace/LRS_linux_compile_lib_ci/src/core/option-interface.h:8,
                 from /home/jenkins/workspace/LRS_linux_compile_lib_ci/src/option.h:5,
                 from /home/jenkins/workspace/LRS_linux_compile_lib_ci/src/source.cpp:6:
/home/jenkins/workspace/LRS_linux_compile_lib_ci/third-party/rsutils/include/rsutils/json-fwd.h:41:17: warning: inline function 'bool rsutils::json_base::exists() const' used but never defined
   41 |     inline bool exists() const;
      |                 ^~~~~~

Moved inline operator bool() to json.h.

Fixed additional Python SyntaxWarning: invalid escape sequence '\ ' warnings for backslash usage with invalid following character.