fix(guardrails): keep auto combos exempt from the vision bridge credential guard (#12373)

getBestVisionModel() validates a configured fixedModel with hasUsableCredentialsForModel() before short-circuiting (#8430). auto / auto/* ids are virtual combos with no provider row, so that check always reported a confirmed false and the combo was silently discarded in favour of global auto-selection — it never got the chance to rotate its members. This mirrors the exemption the reroute guard in visionBridge.ts already carries; concrete fixedModel ids keep the #8430 fall-through unchanged.

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.