[Unit Tests] Better test for `code_extrators::extract_header_implementation`

[bshikin]

ChatGptMixin::extract_header_implementation doesn't test a case when ChatGPT doesn't split header/implementation at all

[just-in-chang]

@bshikin I can take this! By not splitting the header/implementations, do you mean like this (same code block):

// Header.h
{header code}

// Implementation.cpp
{implementation code}

or this (no comments)?

{header code}
{implementation code}

If the latter, what is the intended behavior? should all of the code just be returned in the header?