avoid emitting unhandled rejections on DecompressionStream

[anonrig]

Fixes #6061

DecompressionStream (and more broadly, any pipeThrough + async iteration pattern) generates spurious unhandledrejection events even when the caller properly catches errors. This was caused by three sites creating rejected promises that were never marked as handled:

(1) the pipeThrough error handler in readable.c++ unnecessarily re-wrapped rejections via js.rejectedPromise(), creating a transient unhandled promise before V8's microtask chain could adopt it — fixed by removing the redundant .then() and marking the pipe promise as handled directly;
(2) the async iterator's pushCurrent in iterator.h re-rejected in its error handler, orphaning a promise whose error was already propagated through nextImpl — fixed by swallowing the rejection with .catch_() since pushCurrent is purely bookkeeping;
(3) the async iterator's returnFunction in readable.c++ called reader->cancel() on an already-errored stream without catching the resulting rejection. Fixed by using .catch_() to swallow the expected error during teardown.