Replace raw ReadableStream& ref with jsg::WeakRef

[jasnell]

No description provided.

[ask-bonk]

The change is well-reasoned and correct. Let me verify one more thing -- the old code used addRef() on a raw ReadableStream& which would always succeed, while tryAddRef() on a WeakRef can return kj::none. The callers already store the result in kj::Maybe<jsg::Ref<ReadableStream>> and use the ref purely as an RAII keep-alive guard, so a kj::none result is handled gracefully.

LGTM

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