fix(sse): name the shadowed custom provider node in the no-credentials error (#12365)

When a built-in provider's id or alias reserves the prefix of an existing OpenAI/Anthropic-compatible node — v3.8.50 added openference with alias of, shadowing nodes created earlier with prefix of — the runtime error `No active credentials for provider: openference` gave the operator nothing to act on. It now explains that the prefix routed to the built-in, names the shadowed node, and logs an AUTH warning. Precedence is unchanged and the lookup runs only on the credential-failure path when no connection was tried, so the hot routing path is byte-identical.

Validated in a combined worktree with all 25 PRs of this batch boarded together: typecheck:core clean, 443/443 node-runner tests plus 14/14 vitest across every test file the batch touches, and check-changelog-integrity, check:cycles (418 files), check:provider-consistency (272 REGISTRY entries, 355 canonical providers), check:docs-counts, check:docs-sync (42 locales) and check-file-size all green.

Thanks @pacocartones.