fetchUnifiedEmails, fanOutUnifiedQuery and the cross-account fanOutCrossQuery
stamped accountId/accountLabel/source* directly onto each email object
returned by the per-account client. Those objects are shared references;
mutating them in place could surprise any caller that retained them (and
corrupt an account-state snapshot). Decorate shallow copies instead, at all
three fan-out sites.
The original fix/unified-mailbox-no-mutation branch predated the cross-account
"All accounts" feature and only covered two sites; this re-applies the fix to
main's current code, including the third (shared/group) fan-out site, and
preserves all five stamped fields. Flips the characterisation test to assert
the client's object is left untouched.
