Summary

Builds on #23647 (Stage 1). Adds model-level provider config priority lists so each model can reference multiple provider configs from the same provider family in fallback order. Disabled/unusable configs are skipped at selection time.

Enforces mandatory explicit attachments (Design A): every chat model must be bound to at least one provider config. Models without attachments are hidden from users and rejected at runtime.

Mux working on behalf of Mike.

Database

• Migration 000467: New chat_model_provider_configs join table with model_config_id, provider_config_id, priority, timestamps, and proper constraints/indexes.
• Backfill: Existing models get a priority-0 attachment for their family's currently enabled provider (using ROW_NUMBER() for safe multi-row handling).

API

• ChatModelConfig response now includes provider_configs (ordered attachments with provider metadata, enabled/has_api_key status).
• CreateChatModelConfigRequest requires provider_config_ids (ordered list). Requests without at least one ID are rejected with HTTP 400.
• UpdateChatModelConfigRequest accepts provider_config_ids (null = unchanged, non-null = full replacement).
• Cross-family validation rejects attachments from the wrong provider family.
• Duplicate provider config IDs are rejected.
• Deleting a provider triggers reconcileChatModelProviderConfigsAfterProviderDelete, which re-indexes remaining attachments or soft-deletes models that lose all attachments.

Runtime

• resolveChatModel walks ordered provider attachments instead of merging by provider family name.
• Skips disabled providers and providers without API keys.
• First usable attachment wins. If none usable, model is unavailable.
• Models with zero attachments return an explicit "no explicit provider attachments configured" error.

Cache

• ModelAttachments(ctx, modelID) provides cached attachment lookups via tlru and singleflight.
• Epoch-based invalidation (modelAttachmentEpoch, modelTopologyEpoch) prevents stale singleflight fetches from overwriting newer user updates.

Frontend

• ModelForm: Provider config priority editor with drag-and-drop reorder, status badges, and minimum-one-attachment Yup validation.
• ModelsSection: Provider config count indicator in model list rows.
• ProvidersSection: Shows bound model count per provider.
• modelProviderOptions: Utility for extracting provider option labels and descriptions.
• storyFixtures: Shared test fixtures for Storybook stories.

Tests

• API tests for create with/without configs, cross-family rejection, duplicate IDs, reorder, nil update, shared ProviderConfigIDValidation table-driven cases.
• Runtime tests for attachment-based provider resolution with table-driven TestResolveChatModel.
• Cache tests for attachment lookups, epoch-based stale-write prevention, cascading invalidation.
• Frontend unit tests for form value initialization, ordering, and provider option extraction.