Treat explicit provider model catalogs as authoritative

[copyberry]

Treat explicit provider model catalogs as authoritative

Why

Providers with model_catalog_url could advertise bundled models absent from their catalog or reuse stale models after a failed refresh. Model metadata could also match by prefix or namespaced suffix instead of the provider's exact model ID.

What changed

• Suppress bundled model fallback and merging for explicit provider catalogs, including when discovery is disabled.
• Require unique, non-empty catalog slugs and match metadata by exact model ID.
• Serialize explicit catalog refreshes and invalidate in-memory and persisted catalog entries after fetch failures, preventing stale cache reuse.
• Report a clear configuration error when startup has no available model, while allowing an explicitly configured model.

Testing

Add regression coverage for exact model matching, hidden catalog entries, cache invalidation across restart and recovery, invalid slugs, and empty-catalog startup with and without an explicit model. Update app-server model-list expectations and verify the TUI configuration error.

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