fix: surface CreateProcessW failures as 'exit' instead of uncaughtException on Windows

[codebytere-ant]

Since connect() was deferred to _completePtyConnection() (#885), a failure from CreateProcessW — ERROR_FILE_NOT_FOUND, ERROR_ACCESS_DISABLED_BY_POLICY (1260) under AppLocker/WDAC, etc. — throws from inside the conout worker's onReady callback (or its 5s timeout fallback). Nothing is in a position to catch it: pty.spawn() has already returned, and the outSocket 'close' listener that drives 'exit' isn't attached until ready_datapipe, which never fires. The throw lands as a process-level uncaughtException, the pseudoconsole and worker thread leak, and the consumer's IPty sits with pid === 0 and no onExit.

Before #885 the same throw was synchronous inside the WindowsPtyAgent constructor and propagated out of pty.spawn(), so callers could try { spawn() } catch.

This adds an onError event on WindowsPtyAgent that WindowsTerminal subscribes to in its constructor (i.e., before any async work). On a connect() throw the agent releases the pseudoconsole, disposes the conout worker, destroys both sockets, sets exitCode to the GetLastError() value parsed from the native error string (or -1), and fires the event; WindowsTerminal routes it to 'exit' so the public onExit fires.

Test exercises the path by spawning an existing non-executable file (package.json) — it passes startProcess's existence check, then CreateProcessW fails inside connect() with ERROR_BAD_EXE_FORMAT.

cc @deepak1556

[deepak1556]

Can you address the failing tests, thanks!