feat: add provider-wide model disable flow

[NaNomicon]

Summary

• add provider-wide disabled model storage and mutation flow without changing the flat alias schema
• hide disabled models from /v1/models, reject direct and alias-resolved requests for disabled models, and keep combo execution aligned with absent-model semantics
• add disable/enable controls on the provider page, preserve disabled state across /models import, and exclude disabled models from combo search results

Why

Temporarily stopping use of a provider model currently requires destructive remove/re-add behavior, and imports from /models can reintroduce models in inconsistent ways. This change makes disable state explicit, non-destructive, and enforced consistently across discovery, runtime, UI, import, and combo selection.

Notes

• disabled state is provider-wide rather than account-specific
• disabledModels stores bare model IDs and takes precedence over enabledModels
• the provider-wide disabled list is mirrored across all connections for the provider so newly reactivated connections inherit the same state
• branch includes the combo-search follow-up so disabled models are treated consistently in the combo modal too

[NaNomicon]

Verification note: I validated the provider-disable behavior on the fork branches before splitting this PR. When re-verifying the cherry-picked upstream branch locally, the existing upstream build script (next build --webpack) failed immediately because the installed Next CLI in that environment rejected the --webpack flag. That failure appears to come from the current upstream/base environment rather than this provider-disable diff itself, but I wanted to make the caveat explicit for reviewers.