Error the audio stream when the connection closes before turn.end

[1letme]

Fixes #34.

Problem

onclose ends every in-flight audio stream with push(null) — the same thing the turn.end handler does on a normal finish. So when a connection drops mid-synthesis (before turn.end), the truncated result looks identical to a complete one: toStream() emits a clean end with a partial buffer, toFile() writes a short file, and there's no error to catch and retry on. It shows up most with rotating/residential proxies, whose connections drop mid-stream fairly often.

Change

• Track whether turn.end was received for each request (a turnEnded flag on the stream record).
• In onclose, streams that finished normally still end cleanly; streams closed before turn.end are now destroy()ed with an Error, so callers can catch it and retry.
• toFile(): .pipe() doesn't forward a source-stream error to the writable, so the truncated case would otherwise hang the write promise forever. Added an audioStream error handler that tears down the file stream and rejects. The happy path is unchanged.

Tests

Added a truncated connection suite backed by a local WebSocket server standing in for Edge:

• drops the socket before turn.end → the stream must error (partial audio has already been delivered), and
• sends turn.end normally → the stream must end cleanly (guards against false positives).

Existing tests are unchanged and still pass.

[1letme]

Quick note on the failing check — it's the docs deploy, not the change itself. deploy_docs.yml runs on pull_request, and its last step (Deploy 🚀) force-pushes the built docs to the gh-pages branch, which 403s because a PR build only gets a read-only GITHUB_TOKEN (and from a fork it has no write access here at all):

remote: Permission to Migushthe2nd/MsEdgeTTS.git denied to github-actions[bot].
fatal: unable to access 'https://github.com/Migushthe2nd/MsEdgeTTS.git/': The requested URL returned error: 403

The steps that actually check the code — pnpm run build and npx typedoc — both pass; only the deploy step fails. It should go green once this is on main.