Callers need to know whether a channel is open before create/send, and
had no way to ask without inferring it from a "channel already exists"
error. Exposed at both layers so FFI consumers get the same answer.
"Exists" means currently held by the manager, i.e. between a successful
createReliableChannel and closeChannel. Persisted SDS state outlives a
close, so a closed channel reports false -- this is a liveness check,
not a "was this channel ever created" check.
Closes#4039
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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