gh-121794: Don't set `ob_tid` to zero in fast-path dealloc (#121799)

gh-121794: Don't set `ob_tid` to zero in fast-path dealloc (#121799)

We should maintain the invariant that a zero `ob_tid` implies the
refcount fields are merged.

* Move the assignment in `_Py_MergeZeroLocalRefcount` to immediately
  before the refcount merge.
* Update `_PyTrash_thread_destroy_chain` to set `ob_ref_shared` to
  `_Py_REF_MERGED` when setting `ob_tid` to zero.

Also check this invariant with assertions in the GC in debug builds.
That uncovered a bug when running out of memory during GC.