* fix(imessage): handle stdout/stderr stream errors in the RPC client child
A dead imsg RPC helper can emit an async error on any of its stdio streams.
On a raw stream an unhandled 'error' event throws and surfaces as an
uncaughtException, crashing the gateway. #75438 added this guard for stdin
but left stdout/stderr — on the same long-lived child — unguarded.
Route stdout/stderr stream errors through the existing failAll path via a
shared failFromStreamError helper, mirroring the stdin handler. Add a
regression test that emits 'error' on each stream and asserts the child does
not throw and in-flight requests reject cleanly.
* fix(imessage): terminate failed RPC transports
* test(imessage): exercise real stream failure
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Co-authored-by: Peter Steinberger <steipete@gmail.com>
