Same class of race fixed for newTimerFn in 8e5ef365d: tests swapped the
package-level jsonMarshalFn hook while async goroutines leaked from
earlier tests (sendCommandAck ack retries via runAsync) could still be
reading it, tripping the race detector. Replace the global with a
per-Agent jsonMarshalFn seam (nil defaults to json.Marshal), make the
decode payload helpers Agent methods so they use it, and inject the
failing marshaller into the tests that previously swapped the global.
Verified with go test ./internal/dockeragent/ -race -count=20.
Contract-Neutral: test seam refactor to fix data race, no public contract delta
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
courtmanr@gmail.com