fix(stream-transform): handle promise rejections without leaking

[jakezwang]

Handle a rejected async transform handler in the same then call as its fulfillment. Attaching catch to the original promise leaves the promise returned by then unhandled, so a process can terminate even after the callback or pipeline has received the error.

Adds callback and pipeline regressions in child processes with --unhandled-rejections=strict; both fail before the fix and pass afterward. The original error is preserved.

Validation: stream-transform's 79 tests and type checks pass on Node 20, 22 and 24; package lint and build pass. Full workspace npm test passes on Node 24 (1,047 passing, 6 existing pending). Root lint still reports 5 errors and 6 warnings in unchanged packages, identical to an unmodified-source run.