[roychying]

Why?

The GitHub change provider ignores the URI host and sends every request to its configured API. A URI naming another GitHub instance therefore fails with a misleading error ("could not resolve to a Repository" or "head SHA changed") instead of the validator's clear host rejection, which never runs because the validate stage calls the change provider first.

What?

• Params.Host is required, mirroring the GitHub validator's Params.Host. NewProvider errors when it is missing.
• validateChangeConsistency requires every URI to name that host before any API call, which also rejects a stack spanning hosts (previously only scheme, org and repo were compared).
• The orchestrator server's github change provider block gains host, defaulting to github.com like the validator's.
• The demo GitHub profile's GitHub Enterprise hint includes host.

Breaking: NewProvider now returns (changeprovider.ChangeProvider, error), and callers must pass Host.

Test Plan

• TestValidateChangeConsistency: new cases for a PR on another host, a stack spanning hosts, and a port the provider does not name.
• TestNewProvider: a missing host errors.
• TestProvider_Get: a wrong-host URI errors without any request reaching the server.
• bazel test //submitqueue/extension/changeprovider/... //service/submitqueue/orchestrator/... passes; make lint and make check-gazelle pass.

Issue