feat: allow immediate TaskAction GC

[Rohithmatham12]

Summary

• allow executor GC to treat gc.maxTTL <= 0 as immediate deletion of terminal TaskActions on the next GC cycle
• remove the setup-time validation that rejected maxTTL <= 0 when GC is enabled
• update config/help text and add both envtest coverage and a pure unit test for the TTL decision table

Fixes #7008.

Testing

• GOCACHE=/private/tmp/flyte-go-cache GOMODCACHE=/private/tmp/flyte-go-mod-cache go test ./executor/pkg/controller -run TestShouldDeleteTerminalTaskAction

Note: full go test ./executor/pkg/controller could not run locally because this machine is missing envtest binaries (/usr/local/kubebuilder/bin/etcd). The focused pure unit test above passed; the added Ginkgo spec should run in CI where envtest is available.