Fix Buffer *Write methods to default omitted length to remaining space

[says1117]

What

The encoding-specific Buffer write methods (asciiWrite, base64Write,
base64urlWrite, hexWrite, latin1Write, ucs2Write, utf8Write)
defaulted an omitted length to the full buffer size, then validated it
against the space remaining after offset. Any two-argument call with a
non-zero offset therefore threw:

RangeError [ERR_OUT_OF_RANGE]: The value of "length" is out of range.
It must be >= 0 && <= 9. Received 10

This breaks real callers - notably protobufjs ≥ 7.6.0, which calls
buf.utf8Write(val, pos) during encoding.

Fix

Default the omitted length to this.length - offset in all seven
methods, matching Node.js (which itself hit and reverted this exact bug
class between v22.7.0 and v22.8.0, nodejs/node#54523).

Testing

Adds writeMethodsDefaultLength to buffer-nodejs-test.js, exercising all
seven methods with a non-zero offset and omitted length, plus a byte-level
spot check for utf8Write. The expected results were cross-checked against
Node.js.

Fixes #6875

[github-actions]

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[says1117]

I have read the CLA Document and I hereby sign the CLA

[says1117]

Hi! This is my first contribution, so the internal build workflow didn't run automatically on my fork. Could one of the assigned reviewers kick it off when you have a chance?

Quick summary of the change: the encoding-specific Buffer *Write methods (utf8Write, asciiWrite, etc.) defaulted an omitted length to the full buffer size but validated against the space remaining after offset, so any two-arg call with a non-zero offset threw ERR_OUT_OF_RANGE (breaks protobufjs). Fixed to default to this.length - offset, matching Node.js. Added a regression test covering all seven methods; verified the expected results against Node directly.

Thanks for taking a look! :)