MCP Inspector never declared the skills extension as a client, so strict servers refused every skills call.

PR #2374, merged September 14, fixes the Inspector sending skills/list, skills/get and resources/directory/read once a server declared io.modelcontextprotocol/skills, while never declaring the extension in its own client capabilities (GitHub). The fix adds Skills to ADVERTISABLE_EXTENSIONS in core, plus strict fixtures on ports 3232 and 3233, one per protocol era. Chasing it exposed two more defects: the skills integration test set protocolEra on the transport config, which InspectorClient does not read, so every modern case had been connecting on legacy; and the SDK's 2026-07-28 codec checks resultType then deletes it before a caller's schema runs, so three Inspector schemas requiring it could never pass.