NewPlugin packages the worker side of the integration as a plugin built
on temporal.SimplePlugin: it registers the InvokeModel / ListMcpTools /
CallMcpTool Activities at worker start and closes the cached MCP
toolsets at worker stop, replacing the manual NewActivities + Register +
Close sequence as the standard wiring. Replay run contexts are tracked
so a plugin shared with a workflow replayer never closes the worker's
toolsets (Close is terminal). NewActivities + Register remain for the
test environments, which construct no real worker and therefore run no
plugins.

Cut as v0.2.0.