Logic changes before workflow execution report a null pointer error

### Feature Summary

A logic change can arrive after a workflow is attached but before its execution service is initialized. The WebSocket handler dereferences the missing service and reports a Java null pointer error.

Before: logic change before execution initialization reports a null pointer error.

After: logic change before execution initialization reports that workflow execution is not initialized.

Reproduction evidence:

1. Start the local computing unit service.
2. Open the workflow WebSocket for a workflow that has not started execution.
3. Send a `ModifyLogicRequest`.
4. Observe the `WorkflowErrorEvent` message.

Version and commit evidence:

1.3.0-incubating-SNAPSHOT on main

**Commit Hash (Optional)**
70c21145887920528d7d5540e3fb790b43e8b759

**Relevant log output**

java.lang.NullPointerException: Cannot invoke WorkflowExecutionService.executionReconfigurationService because executionService is null

### Proposed Solution or Design

After: logic change before execution initialization reports that workflow execution is not initialized.

### Affected Area

Workflow Engine (Amber)