#23363 cmux-next: the app test target compiles again (RemoteChildSessionsTests fake gets isReadOnly)

The CmuxNext test target on feat-cmux-next does not compile, so every Swift package test lane is red. #23163 added a fake AgentSessionRemoteClient in RemoteChildSessionsTests without isReadOnly, which #21258 made a protocol requirement:

RemoteChildSessionsTests.swift:14:37: error: type 'RemoteChildSessionsTests.FakeClient' does not conform to protocol 'AgentSessionRemoteClient'
note: protocol requires property 'isReadOnly' with type 'Bool'

The fake now says it can prompt (isReadOnly = false), as the chat it stands in for does.

Changelog

none

Observed / Not run

• Observed: the error above at base c2b90f6d9e04 (fleet step d1279ae1c1ebac4f0c8ec01d, suite SidebarIncrementalMatchesRebuildTests, which never got past the compile).
• Observed: RemoteChildSessionsTests passes on this head (2548f63d7e19, fleet step 2b032a233468, 4 tests).

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