websockets: track bufferedAmount for typed arrays in ArrayBuffer (v1.8.0 backport of #5716)

[mstoykov]

What?

Backport of #5716 to the v1.x release branch for v1.8.0.

Fixes a bug in the WebSocket implementation where bufferedAmount was not
being tracked when sending typed arrays backed by an ArrayBuffer. The
fix correctly accounts for the byte length of these payloads, and a test
is added to cover the case.

Credit to external contributor @prakharbirla-ng for the original change.

Why?

The bufferedAmount property is part of the WebSocket API contract;
users relying on it to backpressure-manage large binary sends would have
observed an incorrect (zero) value when sending typed arrays. Backporting
this small, well-scoped fix into v1.8.0 keeps the v1.x line consistent
with master without pulling in unrelated changes.

Checklist

• I have performed a self-review of my code.
• I have commented on my code, particularly in hard-to-understand areas.
• I have added tests for my changes.
• I have run linter and tests locally (make check) and all pass.

Checklist: Documentation (only for k6 maintainers and if relevant)

Please do not merge this PR until the following items are filled out.

• I have added the correct milestone and labels to the PR.
• I have updated the release notes: link
• I have updated or added an issue to the k6-documentation: grafana/k6-docs#NUMBER if applicable
• I have updated or added an issue to the TypeScript definitions: grafana/k6-DefinitelyTyped#NUMBER if applicable

Related PR(s)/Issue(s)

• Backport of websockets: buffered amount not tracked in ArrayBuffer for typed arrays - fixes #5715 #5716
• Original issue: WebSocket bufferedAmount isn't tracked for TypedArrays #5715