feat(core): action output normalization

Fixes #9999.

Note TODO items:
- [ ] Renormalize cached_context across action boundary. Blocked by #10369.
- [ ] Add extra tests for surrogate pairs
- [ ] Move set_context_from_string into helper module
- [ ] if we don't apply normalization, we still need to fixup the
      app_context, to keep it coherent with cached_context (or at least
      we need to verify that app_context is never used in this
      situation)