Fix overlooked case in erc--get-inserted-msg-beg-at

Fix overlooked case in erc--get-inserted-msg-beg-at

* lisp/erc/erc.el (erc--get-inserted-msg-beg-at): Account for the start
of a props header being `bobp' when searching backwards.
(erc--get-inserted-msg-prop): Add optional `point' parameter.
* test/lisp/erc/erc-goodies-tests.el
(erc--get-inserted-msg-beg/truncated/readonly): New test.
* test/lisp/erc/erc-tests.el (erc--get-inserted-msg-beg/truncated): New
test.
* test/lisp/erc/resources/erc-tests-common.el
(erc-tests-common-assert-get-inserted-msg/truncated): New test helper.
(Bug#72736)

(cherry picked from commit 08f662da1123f50f3f62d0af747bff88d2c73938)