[plugins] Phase 5d: serve plugin catalog surfacing types.Plugin (THV-0077)

[JAORMX]

Goal: Serve the plugin catalog from toolhive-registry-server, surfacing types.Plugin — mirroring how skills are handled.

Deliverables:

• After bumping toolhive-core to the C3 tag, implement provider/catalog endpoints surfacing registry/types.Plugin, mirroring the existing skills handling.

Dependency/gate: depends on GATE-C3 (toolhive-core registry/types.Plugin tagged + go.mod bumped in both toolhive and toolhive-registry-server). Parallel with Phase 5b/5c.

Testing/exit gate: catalog list/get for plugins surfacing types.Plugin.

Part of stacklok/toolhive#5525
Refs RFC PR stacklok/toolhive-rfcs#77