Preserve ERC prompt and its bounding markers

Preserve ERC prompt and its bounding markers

* lisp/erc/erc.el (erc--assert-input-bounds): Add possibly temporary
helper function to sync `process-mark' to `erc-insert-marker' in
server buffer.
(erc-display-line-1): Expect `erc-insert-marker' to always be
initialized.  Assert some essential invariants regarding insert
markers.
(erc-send-current-line): Delete typed input but not the prompt.
(erc-display-msg): Rework slightly to respect existing markers.
* test/lisp/erc/erc-dcc-tests.el
(erc-dcc-tests--dcc-handle-ctcp-send): Set insert marker.
* test/lisp/erc/erc-networks-tests.el
(erc-networks--rename-server-buffer--existing-live): Initialize
markers to appease `erc--assert-input-bounds'.
* test/lisp/erc/erc-tests.el (erc-ring-previous-command): Fix sloppy
mock.  (Bug#60936.)