`streamAnthropic` runs for every `anthropic-messages` provider, not just
GitHub Copilot. The previous carve-out in `isProviderRetryableError`
checked only `status === 400 && code === "model_not_supported"`, which
means a genuinely-unsupported model on a non-Copilot Anthropic-compatible
backend would have been retried 3x with 2s/4s/8s backoff before failing.
Add `provider?: string` parameter and gate the carve-out on
`provider === "github-copilot"`. Callsite in `streamAnthropic` now
passes `model.provider`. Existing in-request retry wrappers in
`openai-responses.ts` / `openai-completions.ts` were already provider-
gated via `callWithCopilotModelRetry`, so no change there.
Add a regression test asserting the predicate returns true only when
`"github-copilot"` is passed.
Addresses Codex review feedback on #722.
