Both SSE handlers installed their subscriber, primed the stream so
Firefox's fetch() resolves (#1858), and only then registered onAbort.
Hono's onAbort has no already-aborted replay and abort() fires only the
subscribers registered at that moment, so a client disconnecting while
the priming write was in flight fired nothing: the session event
consumer / server-list subscriber stayed installed, the mcp.json watcher
was never stopped, and the handler parked forever on a promise nothing
would resolve.
Add primeAndHoldSseStream, which registers one listener before the first
await — it both cleans up and releases the hold, collapsing the previous
duplicate onAbort registrations. Priming still happens after the
subscriber is installed, since callers treat the stream's first bytes as
proof they are subscribed.
Closes#1999
Signed-off-by: cliffhall <cliff@futurescale.com>
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