fix: stop fake protoc child recursion

[ryanfowler]

Summary

• dispatch the fake protoc child worker from its command-line argument before checking the inherited test mode
• prevent recursive child creation and concurrent PID-file truncation in the Windows containment test
• preserve the intended inherited-pipe containment coverage

Testing

• go test -v ./internal/proto
• go test -parallel 8 ./...
• GOOS=windows CGO_ENABLED=0 go test -c ./internal/proto
• git diff --check