fix(memburst): validate sampling config

[pkurunner]

Summary

• validate MemoryBurst sliding window, interval, and process dump limits before starting
• return a clear error instead of panicking on zero or negative user config
• add table-driven coverage for the validation helper

Testing

• gofmt -w core/autotracing/memburst.go core/autotracing/memburst_config_test.go
• Not run: GOOS=linux GOARCH=amd64 go test -c ./core/autotracing is blocked on a clean upstream snapshot by missing generated internal/toolstream/transport capnp types.