[Davien21]

Context

encodeInto() was writing into a Uint8Array even when its buffer came from transferToImmutable(), so it changed memory that's supposed to be read-only.

Now it throws ERR_INVALID_ARG_VALUE and the bytes are left alone.

Fixes: #66346 This is part of the immutable ArrayBuffer audit.

The check is in the C++ binding, before either write path, so the short-string path and the fast path for long strings are both covered. When the buffer is immutable the binding returns false and the JS side throws, with the same error code and message as websocketMask().

One thing I wasn't sure about: an empty string into a zero-length immutable view also throws, rather than quietly doing nothing. I went with consistency, but I'm happy to change it if you'd rather it be a no-op?

I added test/parallel/test-whatwg-encoding-custom-textencoder-immutable.js, which tries empty, short, accented, long, multi-byte and lone-surrogate strings against whole, offset and zero-length views. I built Node locally on macOS (arm64), and tools/test.py test/parallel/test-whatwg-encoding-* passed, as did make lint-js lint-cpp.

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