ModifyLogic before execution initialization fails with a null pointer

### Feature Summary

Root cause: the WebSocket handler checks that a workflow is attached, then dereferences an execution service that can still be null.

Before: a workflow is attached but execution has not started, then ModifyLogicRequest throws a NullPointerException.

Expected: the handler reports that workflow execution is not initialized and sends a WorkflowErrorEvent.

Reproduction evidence:

1. Attach a WebSocket session to a workflow without starting execution.
2. Send a ModifyLogicRequest frame.
3. Observe the NullPointerException.

A regression test against commit caeb27ea54 reproduced this with 15 passing tests and 1 failing test.

Version and commit evidence:

1.3.0-incubating-SNAPSHOT on main

**Commit Hash (Optional)**
caeb27ea54

## Browsers

Not browser specific.

## Relevant log output

java.lang.NullPointerException

### Proposed Solution or Design

Expected: the handler reports that workflow execution is not initialized and sends a WorkflowErrorEvent.

### Affected Area

Workflow Engine (Amber)