[Port to 2.x]: feat: support partial results in SHOW MEASUREMENTS (#27443)

Port master-1.x commit https://github.com/influxdata/influxdb/commit/4300c5b650 to main-2.x.

Wildcard SHOW MEASUREMENTS ON *.* returns partial rows plus warning messages instead of failing on the first errored source. 2.x's v1/coordinator/statement_executor.go resolves sources via DBRP mappings, so this is a semantic port rather than a cherry-pick.