Add an agent-facing update tool for MCP servers #1849

mcp.addServer has an agent-facing counterpart for remove (integrations.remove, #1600), but updates still require the HTTP API or UI (POST /mcp/servers/:slug/config and /auth, plus the edit flows from #1775/#1821). There is no agent-facing mcp.updateServer / configureAuth tool, and a remote addServer without authenticationTemplate still defaults to auth none.

Split from #1330 (part 2 residual). This is an API-surface decision, not a bug — the remove+re-add path works today.