Stabilize file store suffix expiry test

[asim]

Summary

• Stabilize the file store suffix expiry coverage by replacing exact TTL-boundary sleeps with deterministic expired/unexpired records.
• Keep suffix filtering assertions focused on live records and delete behavior without depending on slow CI timing.

Testing

• go build ./...
• go test ./... (fails in this sandbox because loopback gRPC targets are forbidden by the environment)
• golangci-lint run ./...
• go test -race -cover ./store -run TestFileStore -count=1

Closes #3780
Closes #3827