Preserve signal payload integer precision in workflow.SignalWorkflow

[Copilot]

workflow.SignalWorkflow could silently corrupt large integers (>2^53) because the system activity accepted interface{}, forcing JSON numbers through float64 during activity argument decoding. This changed payload values before they reached typed signal channels.

• Root cause addressed: remove lossy interface{} hop

  • internal/signals.Activities.DeliverWorkflowSignal now accepts json.RawMessage instead of interface{} for the signal argument.
  • This preserves raw payload bytes across activity serialization/deserialization and avoids float64 coercion.

• Signal API path updated to preserve typed payload encoding

  • workflow.SignalWorkflow[T] now serializes arg with the workflow converter up front and passes the result as json.RawMessage to the system signal activity.
  • Conversion failures now include signal/workflow context in the returned error.

• Regression coverage for high-magnitude integers

  • Added a test in tester/tester_test.go that sends a payload containing uint64(123456789012345678) and asserts exact round-trip equality after SignalWorkflow.

input, err := contextvalue.Converter(ctx).To(arg)
if err != nil {
    // contextual conversion error
}

return ExecuteActivity[any](ctx, opts, a.DeliverWorkflowSignal, instanceID, name, json.RawMessage(input))