Fix right-sided stamps commingling with erc-prompt

Fix right-sided stamps commingling with erc-prompt

* lisp/erc/erc-stamp.el (erc-insert-timestamp-left-and-right): Fix bug
that saw the prompt being inserted after messages but just inside the
narrowed operating portion of the buffer, which meant remaining
modification hooks would see it upon visiting.  Thanks to Corwin Brust
for catching this.
* test/lisp/erc/erc-fill-tests.el (erc-fill-wrap--monospace): Use
custom `erc-prompt' function to guarantee invariants asserted by
`erc--assert-input-bounds' are preserved throughout.  (Bug#60936)