(decode_coding): Avoid nested *-change-functions (bug#78042)

(decode_coding): Avoid nested *-change-functions (bug#78042)

* src/coding.c (decode_coding): Avoid nested *-change-functions (bug#78042).

* test/src/editfns-tests.el (sanity-check-change-functions-before)
(sanity-check-change-functions-after): Record notifications in
`sanity-check-change-functions-op`.
(sanity-check-change-functions-with-op): Don't rely on
`sanity-check-change-functions-op` always holding only the `op`.
(sanity-check-change-functions-errors): Include the sequence of
notifications in the error info.
(editfns-tests--before/after-change-functions): Add tests for (bug#78042).

(cherry picked from commit 9048fcf22c3ec3b5cc77dbb98993a53f5f9b7cd5)