fix(core): surrogate handling, markers in app context, and memory leak

Fixes #10615 (surrogate handling, km_core_state_context_set_if_needed).
Fixes #10616 (markers in app context, in unit tests)
Fixes #10617 (memory leak in replace_context).

The surrogate handling and markers in app context bugs were quite
tangled, so opted to fix those both together in one commit. The memory
leak was spotted while verifying the changes for surrogate handling.

Surrogates were not handled in is_context_unchanged, and opted to
refactor to work directly with km_core_context_item arrays rather than
continue with the string-based comparison, for simplicity.

While writing the corresponding unit tests, realised that the
app_context checks were incorrect in a number of existing tests, and so
fixed up the existing tests at the same time as writing up the new
tests. The only changes to core for this were to add assertions for the
app_context to ensure that markers are not inadvertently added.