Fix three child-process stdin write paths that let async EPIPE errors
escape to uncaughtException and crash the gateway.
extensions/imessage/src/client.ts (the actual #75438 crash path):
- Add child.stdin.on('error') listener in start() to catch async EPIPE
and reject all pending requests via failAll().
- Add write callback to request() stdin.write() that rejects the
specific pending request on error, instead of leaving it hanging
until timeout.
src/agents/mcp-stdio-transport.ts:
- Fix write callback race in send(): previously resolved the promise
immediately when write() returned true, then the write callback with
EPIPE would fire after the promise was already fulfilled. Now always
settles the promise from the write callback so the outcome is known
before resolving.
src/process/exec.ts:
- Add stdin.on('error') before writing input so EPIPE from a
prematurely-exited child is swallowed — the process exit handler
reports the real status.
One reporter observed a gateway crash after 10.5 hours of stable
uptime — a single EPIPE on an iMessage RPC child process stdin write
killed the gateway with code 1.
Fixes: #75438
