test(worker): cover the Sent view's matched-message branch

Asserted by hand until now, which meant asking the user to look at the
one branch I had rewritten twice after misreading the libnotmuch API.

Both directions in one test: a query matching only the reply reports the
reply under withRecipients, and the same thread under an ordinary query
still reports its opening message, so this is a Sent special case rather
than a change of meaning everywhere.

Mutation checked: disabling the branch makes a Sent row report
a1@example.org, the thread's opener, where a2@example.org is the message
that matched.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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