feat(compute/serve): allow local WebSocket passthrough to be disabled

[harmony7]

Change summary

Adds a way to disable Viceroy's local WebSocket passthrough from fastly compute serve, for cases where it isn't supported or isn't wanted.

There are two controls, both defaulting to enabled:

• a new [local_server.websockets_passthrough] section in fastly.toml with an enable key, mirroring the existing [local_server.pushpin] section
• a new --experimental-websockets-passthrough flag on compute serve, which takes precedence over the manifest (so --no-experimental-websockets-passthrough disables it, and the flag can also re-enable it over a manifest enable = false)

Viceroy already enables WebSocket passthrough by default and that isn't expected to change, so we only pass --enable-local-websocket-passthrough=false when the effective value is false, and pass nothing at all otherwise. That keeps the invocation unchanged for everyone who doesn't opt out.

Resolution order in serve.go:

1. --experimental-websockets-passthrough, if set
2. local_server.websockets_passthrough.enable, if set
3. true

All Submissions:

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

New Feature Submissions:

• Does your submission pass tests?

Changes to Core Features:

• Have you written new tests for your core changes, as applicable?
• Have you successfully run tests with your changes locally?

[local_server.websockets_passthrough] was added to pkg/manifest/testdata/fastly-viceroy-update.toml so the existing TestManifestPersistsLocalServerSection round-trip test covers the new section, and the new flag was added to the ignore list in the compute build / compute serve flag-parity test. go build ./..., gofmt, go vet, and the pkg/app, pkg/manifest, and pkg/commands/compute test packages all pass locally.

User Impact

No change unless a user opts in: with neither the flag nor the manifest section set, the Viceroy command line is byte-for-byte what it was before. Users who need to turn passthrough off now have a supported way to do it.

Are there any considerations that need to be addressed for release?

No breaking changes.

This feature requires fastly/Viceroy#678 to be shipped first. Also worth noting that --enable-local-websocket-passthrough is only ever passed when a user explicitly disables passthrough, so existing setups can't hit that.

[harmony7]

This feature requires fastly/Viceroy#678 to be shipped first.