Align registry API with toolhive-registry-server v0.1 surface

Summary

Make toolhive's registry API a mirror of the v0.1 surface that stacklok/toolhive-registry-server exposes. This:

• Drops the legacy-shaped /api/v1beta/registry/... endpoints.
• Refactors the /registry/{registryName}/v0.1/servers handler to consume toolhive-core/registry/types directly (same pattern pkg/api/v1/registry_v01_skills.go already uses — no converters import).
• Adds the per-version sub-resources that registry-server exposes but toolhive doesn't yet (servers and skills both).

Phase 1 (parse-side internals + error-code cleanup) is stacklok/toolhive#5278.

Reference: registry-server's v0.1 surface

GET /registry/{registryName}/v0.1/servers
GET /registry/{registryName}/v0.1/servers/{serverName}/versions
GET /registry/{registryName}/v0.1/servers/{serverName}/versions/{version}
GET /registry/{registryName}/v0.1/x/dev.toolhive/skills
GET /registry/{registryName}/v0.1/x/dev.toolhive/skills/{namespace}/{name}
GET /registry/{registryName}/v0.1/x/dev.toolhive/skills/{namespace}/{name}/versions
GET /registry/{registryName}/v0.1/x/dev.toolhive/skills/{namespace}/{name}/versions/{version}

Source of truth: stacklok/toolhive-registry-server docs/thv-registry-api/swagger.yaml.

Scope

Remove:

• pkg/api/v1/registry.go v1beta servers handlers (/api/v1beta/registry/{name}/servers, /{serverName}) and the legacy []*registry.ImageMetadata / []*registry.RemoteServerMetadata response structs.
• pkg/api/v1/registry_v01_servers.go /versions/latest shortcut (replace with the version paths below).

Refactor:

• pkg/api/v1/registry_v01_servers.go — drop the toolhive-core/registry/converters import; consume upstream types directly, mirroring pkg/api/v1/registry_v01_skills.go.
• Internal registry provider chain — return upstream-shaped types instead of ImageMetadata / RemoteServerMetadata.

Add (to match registry-server surface):

• GET /registry/{registryName}/v0.1/servers/{serverName}/versions
• GET /registry/{registryName}/v0.1/servers/{serverName}/versions/{version}
• GET /registry/{registryName}/v0.1/x/dev.toolhive/skills/{namespace}/{name}/versions
• GET /registry/{registryName}/v0.1/x/dev.toolhive/skills/{namespace}/{name}/versions/{version}

Coordination

• FE counterpart: studio currently consumes /api/v1beta/registry/{name}/servers and models RegistryItem around RegistryImageMetadata / RegistryRemoteServerMetadata. It needs to migrate to the new v0.1 surface — tracked separately.

Blocks

• stacklok/toolhive-core#106 (legacy Go types and registry/converters/ removal)

Acceptance criteria

• /api/v1beta/registry/{name}/servers and /{serverName} removed.
• No toolhive code imports toolhive-core/registry/converters.
• No toolhive code references registry.ImageMetadata / registry.RemoteServerMetadata.
• toolhive's v0.1 endpoint set matches registry-server's swagger.
• OpenAPI spec regenerated; task docs green.

References

• Pattern reference (already aligned): pkg/api/v1/registry_v01_skills.go
• Surface reference: stacklok/toolhive-registry-server docs/thv-registry-api/swagger.yaml
• Phase 1: stacklok/toolhive#5278
• Landed: stacklok/toolhive-catalog#1215, stacklok/toolhive-core#105, stacklok/toolhive#5273
• Downstream blocker: stacklok/toolhive-core#106

Start with pkg/api/v1/registry_v01_servers.go and the matching pattern in pkg/api/v1/registry_v01_skills.go, then compare the routes with toolhive-registry-server's docs/thv-registry-api/swagger.yaml. Trace the internal registry provider chain and the legacy handlers in pkg/api/v1/registry.go. Done means the v0.1 endpoints match the reference, legacy types and converter imports are gone, and regenerated OpenAPI passes task docs.