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

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PR #2374, merged 2026-09-14, 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, which a strict server reported against Tachyon refused outright. The fix adds a Skills entry to ADVERTISABLE_EXTENSIONS in core/, so the CLI and TUI declare it too, plus new 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, which meant three Inspector schemas requiring it could never pass.