Both branches turn on where Stalwart advertises a capability, so they meet
in the same two files. The mock keeps `urn:stalwart:jmap` out of the
session-level capabilities and hands it out per-account, as a real server
does, while the submission capability it grew for scheduled send lives
per-account beside it; the client keeps both accessors, one asking whether a
capability is advertised anywhere and one reading the object itself.
jcoffey-dev
ca2c71e858