stream: validate options.signal in iter Writer methods

[SAY-5]

Fixes: #64384

The push(), broadcast() and fromWritable() writers all accept an
options.signal on write(), writev() and end(), but none of them
validated it. A malformed value (for example a string) was silently
accepted: on the sync fast path a truthy non-signal skipped straight into
the async path and threw later, end() ignored it entirely, and the
classic writer never looked at it at all. The constructors already run
validateAbortSignal, so the per-write methods were the only place where a
bad signal slipped through.

This validates options.signal at the start of each writer method, matching
the constructor behaviour and throwing ERR_INVALID_ARG_TYPE consistently.
A valid signal and an absent signal are unaffected. For the classic writer
the signal is still not used for cancellation (that is handled at the
pipeline level), it is only validated.

Added assertions to test-stream-iter-validation.js covering all three
writers.

[nodejs-github-bot]

Review requested:

• @nodejs/streams

[trivikr]

The issue https://www.github.com/nodejs/node/issues/64384 is already assigned, and there's an open PR which is linked as well as approved https://www.github.com/nodejs/node/pull/64385