FEAT GUI: Expose built-in dataset provider metadata

Start with SeedDatasetProvider, SeedDatasetMetadata, provider metadata, and the dataset backend service/models; review the identity/summary contract and catalog from items 1 and 3. Inspect existing name-list behavior and the dataset/Python/model/test instructions before tracing the explorer catalog. Done means read-only loaded, provider-only, and combined views preserve provenance, handle missing metadata safely, avoid remote loading, and include compatibility and metadata tests.

Is your feature request related to a problem? Please describe.

The explorer should help users discover built-in datasets that are not loaded yet. Provider names alone do not explain modalities, size, variants, sources, or whether credentials and gated access are required.

Work item 9 of 17 in #2744. Technical prerequisites: the dataset identity/summary contract and catalog from items 1 and 3. This starts the provider-loading milestone after item 8; keep not ready yet until promoted.

Describe the solution you'd like

Add metadata-only descriptors for registered built-in dataset providers and integrate them with the existing dataset catalog contract.

• Distinguish in-memory datasets from available providers and expose a loaded/available filter without confusing name collisions or partial/variant loads.
• Include available description, source URL, license/provenance, modality, harm categories, size information, and supported variants/options. Missing information remains unknown.
• Expose only documented, supported non-secret load options. Use registered provider identities; do not accept arbitrary Python paths, code, or unvalidated kwargs from clients.
• Describe credential and external gated-access prerequisites without returning credential values. A configured credential is not proof that its account has dataset access.
• Keep enumeration side-effect free: no dataset materialization, downloads, model calls, or credential-dependent fetches merely to display cards.

Acceptance criteria:

• Loaded-only, provider-only, and combined views distinguish provenance and availability correctly.
• Known provider metadata is displayed without confusing an estimate or coarse size category with an exact loaded count.
• Missing metadata or an unsupported load option does not break the whole catalog.
• Gated/public examples can be described without real credentials or remote fetches.
• Existing name-list clients remain compatible and metadata/read-only tests are included.

Describe alternatives you've considered, if relevant

Avoid a hardcoded frontend dataset list, a second provider registry, fetching every dataset for metadata, or a generic code-execution/loader endpoint. This issue does not load datasets; item 10 adds that operation.

Additional context

Start with SeedDatasetProvider, SeedDatasetMetadata, provider metadata, and the dataset backend service/models. Preserve the framework rule that downstream consumers read seeds from memory after loading. Coordinate count provenance with #2735 and follow dataset/Python/model/test instructions.