Add SSE fallback for the new WebSocket API endpoints

[EhabY]

In #580, we got rid of the two SSE connections (workspace watch and agent watch-metadata). However, this requires users to update their Coder Server to >=2.22.0 or otherwise would fail with 404. We should instead fallback to SSE when we get a 404 for those two requests so users don't get broken watching behavior simply because they use a new VS Code extension.