test(paper): cover the services reading the live message formats

Swapping LunaticChatConfiguration for MessageFormatHolder pulled
ChannelMessageHandler and CrossServerChatManager into the patch as
uncovered, because neither class had ever had a unit test: nothing
guarded the claim that /lc reload reaches channel chat and cross-server
chat without a restart.

Both now assert that a format replaced at runtime applies to the next
message, alongside the delivery rules they already relied on. The
reloader gains the read failure that carries no message, so the reason
it reports is known not to come out empty.

Co-Authored-By: Claude <noreply@anthropic.com>

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