Only support native ReadableStream streams #4293

Support plan

• is this issue currently blocking your project? (yes/no): no
• is this issue affecting a production system? (yes/no): no

Context

• node version: 14+
• module version: 20.x
• environment (e.g. node, browser, native):
• used with (e.g. hapi application, another framework, standalone, ...):
• any other relevant information:

Handlers currently support returning any ReadableStream, even v1 streams from the readable-stream module. However using such legacy streams is incompatible with modern streams and requires very careful integration to support.

What problem are you trying to solve?

Simpler, more robust internal stream handling. Eg. we can rely on all streams having the .destroy() method and remove tests like:

Do you have a new or modified API suggestion to solve the problem?

Drop node v12 support (#4279) and fail when the returned stream matches value instanceof Stream && !(value instanceof ReadableStream). The current detection logic relies on duck typing.

One drawback is that we increase the reliance on the node runtime, which means that the Chrome "support" as introduced in #3946 will be gone (unless they find a way to properly polyfill ReadableStream). I don't know why this was patched in, since we don't do any kind of testing on that platform.

For a more graceful approach, we could pipe() legacy streams through a Passthrough stream when returned from a handler. Then internal processing could still rely on the stream being native.