The activity tables don't allow internal statements to be written to them. When users turn on the cluster setting sql.stats.response.show_internal.enabled they would expect to see internal statements returned in their sql stats response but will not see any if we service requests using the activity table. The CombinedStatementStats api chooses to service requests using the activity tables as its first choice but if sql.stats.response.show_internal.enabled is true we should disallow the activity tables from being used to service the request.

Jira issue: CRDB-33518