Use WebSocket check in pac-proxy-agent

[addaleax]

This mirrors the check in the core proxy-agent package and serves the same purpose.

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🦋 Changeset detected

Latest commit: 932d9b4b60abf941cd3b61770dd904c5ce811bdc

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

| Name | Type | | --------------- | ----- | | pac-proxy-agent | Patch |

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Name	Status	Preview	Updated (UTC)
proxy-agents	✅ Ready (Inspect)	Visit Preview	Aug 12, 2024 11:00am