Add E2E test coverage for preview URL routing

The `proxyToSandbox()` → `sandbox.fetch()` path (preview URL routing for WebSocket upgrades) has no E2E test coverage. A `this` binding bug in the `getSandbox()` proxy went undetected because:

- Unit tests mock the stub, so `this` binding issues don't manifest
- Existing WebSocket E2E tests use `wsConnect()` (already in `enhancedMethods`), not preview URLs

A unit test for the proxy contract has been added, but an E2E test that actually hits a preview URL would catch routing, token validation, and port switching issues end-to-end.

**Blocked on**: The E2E test worker runs on `.workers.dev` which doesn't support `exposePort()`. The test infrastructure needs a custom domain with wildcard DNS before this test can be added.