copy-paste: Fix edge cases on message selection for copying.
copy-paste: Fix edge cases on message selection for copying.

When we detect that multiple messages are selected, copying will copy the
entirety of each message that is selected. [more conversation on that
here](https://chat.zulip.org/#narrow/stream/6-frontend/topic/triple.20click.20.2B.20copy.20paste.20bug/near/1398292).

This commit fixes a bug where we would select the entirety of a message
following a selection even when none of that last message's text was
selected (only some HTML element in the messages's container).

Fixes #18706.

zulip/zulip,rht/zulip,andersk/zulip,zulip/zulip,andersk/zulip,rht/zulip,zulip/zulip,andersk/zulip,zulip/zulip,rht/zulip,zulip/zulip,rht/zulip,andersk/zulip,rht/zulip,rht/zulip,andersk/zulip,zulip/zulip,andersk/zulip,andersk/zulip,rht/zulip,zulip/zulip