Implement test run retention cleanup

[damilolaedwards]

Summary

cleanupTestHistory was a stub that never removed anything, so every finished test run stayed in the in-memory run map for the life of the coordinator process. On a deployment that runs cron or startup-scheduled tests continuously, this pins an ever-growing set of schedulers, task states, and buffered loggers with no bound, and makes the testRetentionTime setting a no-op regardless of what it's set to.

This change prunes runs whose status is terminal (success, failure, aborted, or skipped) once they are older than the configured retention window, leaving pending and running tests untouched. Evicting a run only drops the in-memory reference; persisted results are unaffected. Coordinator.GetTestByRunID already falls back to loading a run straight from the database when it is missing from the in-memory map, so a run's detail page and logs keep working the same way after eviction, just served from the database instead of memory.

Test plan

• Added a test that seeds the run map with old finished runs, a recent finished run, a running run, and a pending run, then asserts only the old finished ones are evicted.
• Added a test confirming a zero retention window still evicts an already-finished run rather than being treated as disabled.
• go build ./...
• go vet ./...
• go test ./pkg/assertoor/...