kvcoord: verify that distributed processors never read outside expected spans

With #147853, we construct an interval tree for leaf transactions that captures the set of spans from which the leaf txn might read. This allows a reduced set of writes to be shipped with the leaf txn by the write pipeliner and write buffer. We should add some test-build-only validation that processors never read outside those spans. It may be helpful to construct the read tree and validate even for root txns that haven't performed writes in order to increase test coverage.

Jira issue: CRDB-53095