feat(core): adds `km_core_state_context_debug`

Fixes #10365.

Adds a `km_core_state_context_debug` function that returns a km_core_cp
string containing the current cached or intermediate context. The
function must be used only for debug logging purposes -- the string is
not intended to parsed for other purposes and the format may change in
the future.

This commit adds the function and corresponding unit tests. Subsequent
commits will update existing context debug functions in various engines
to use this API endpoint instead.