Refactor duplicated code in callback handling

- Merge create_suspended_state() and create_suspended_state_from_existing()
  into a unified create_suspended_state_ex() with source parameter

- Add cleanup_suspended_state_partial() helper for consistent error cleanup

- Extract build_pending_callback_exc_args() to build exc_args tuple from
  thread-local pending callback state (eliminates ~50 lines of duplicated
  code across 4 call sites)

- Extract build_suspended_result() to build the {suspended, ...} result
  term from a suspended state

- Remove now-unused make_suspended_term() function

- Add forward declarations in py_nif.c for cross-module helper functions

Author: benoitc