External workflow signal from a workflow code throws incorrect exceptions

Actual Behavior

fun signalWorkflow(workflowId: String, signalName: String, vararg args: Any) {
        try {
            Workflow.newUntypedExternalWorkflowStub(workflowId).signal(signalName, args)
        } catch (ex: ApplicationFailure) {
            if (ex.type == "SIGNAL_EXTERNAL_WORKFLOW_EXECUTION_FAILED_CAUSE_EXTERNAL_WORKFLOW_EXECUTION_NOT_FOUND") {
                logger.info {
                    it.message("Workflow not found while attempting to send signal to it.")
                        .value("workflowId", workflowId).value("signal", signalName)
                }
            }
        }
    }

The code above shouldn't work and ApplicationFailure is an incorrect exception to be thrown here.

Additional context

There is already an exception for this situation: SignalExternalWorkflowException that is currently not created anywhere.

There is also a todo to implement exactly this that never was done: https://github.com/temporalio/sdk-java/blob/37a0e463fae2312882b5c946f423a0dbf85f9666/temporal-sdk/src/main/java/io/temporal/internal/statemachines/SignalExternalStateMachine.java#L152

Another alternative exception to consider here may be WorkflowNotFoundException

Start in temporal-sdk/src/main/java/io/temporal/internal/statemachines/SignalExternalStateMachine.java at the TODO around line 152, then inspect SignalExternalWorkflowException and WorkflowNotFoundException. Trace the external signal failure path and determine which exception should represent a missing target instead of ApplicationFailure. Done means the signal path creates and propagates the selected exception consistently.