session-description-handler.spec.ts: Fix flaky NotReadableError

[23Skidoo]

The Web SessionDescriptionHandler interop specs fail intermittently with NotReadableError: Could not start audio source, landing on a different spec each time and more often on loaded machines.

Chrome coalesces getUserMedia calls for one device onto a shared device session. Every spec boundary in this file stops all tracks, dropping the session's consumer count to zero and starting an asynchronous teardown in the audio service. The next spec's getUserMedia is then occasionally told the device is already open and joins the dying session; the track start lands inside the teardown and fails. Chrome's verbose log shows the close and the failure on the same session id, 1.4ms apart:

MSM::Closed({stream_type=DEVICE_AUDIO_CAPTURE}, {session_id=42FA...})
PLAS::OnCaptureError({code=0, message=Maximum allowed input device limit reached or an OS failure occurred.}) [session_id=42FA...]
UMP::OnTrackStarted({request_id = 260}, {session_id=42FA...}, {result=TRACK_START_FAILURE_AUDIO})

A single audio track acquired in beforeAll and released in afterAll keeps the consumer count above zero for the file's whole run, so no teardown ever races a join. In an environment reproducing the failure on roughly two runs in five, ten consecutive runs with this change came back clean.

---

This contribution was prepared with LLM assistance (Claude): the investigation, the change, and this description. A human reviewed it and stands behind the submission.