ConductorWorkflow requires a live executor at construction time — prevents offline use

## Summary

Tested against: **Conductor OSS 3.32.0-rc.9**

`ConductorWorkflow.__init__()` takes `executor: WorkflowExecutor` as a mandatory
first positional argument. This forces a live server connection at the point of
constructing a workflow definition, making offline use impossible.

## Impact

- **Unit testing**: tests that validate workflow structure (task order, task types,
inputParameters) cannot be written without a running Conductor server.
- **Library code**: functions that return workflow definitions must thread an
`executor` parameter through all their call chains, coupling definition logic
to connection management.
- **Code generation / serialization**: workflow definitions cannot be built and
exported to JSON without a server connection.

## Reproduction

```python
# Fails without a server running:
wf = ConductorWorkflow(name="my_wf", version=1)
# TypeError: ConductorWorkflow.__init__() missing 1 required positional argument: 'executor'

# Required:
config = Configuration(server_api_url="http://localhost:8080/api")
executor = WorkflowExecutor(config)
wf = ConductorWorkflow(executor=executor, name="my_wf", version=1)
```

## Suggested fix

Make `executor` optional; raise `RuntimeError` only when an executor-dependent
method (`.register()`, `.start_workflow()`, `.execute()`) is called on a workflow
with no executor set:

```python
def __init__(self, name: str, version: Optional[int] = None,
executor: Optional[WorkflowExecutor] = None, ...):
self._executor = executor
...

def register(self, overwrite: bool):
if self._executor is None:
raise RuntimeError("ConductorWorkflow.register() requires an executor. "
"Pass executor= at construction time.")
...
```

## Verified against

Conductor server **3.32.0-rc.9**, Python SDK (editable install from
`conductor-oss/python-sdk` main branch).

Tìm ConductorWorkflow.__init__(), sau đó kiểm tra register(), start_workflow() và execute() cùng với cách xử lý WorkflowExecutor hiện có. Tái hiện việc khởi tạo mà không có server, sau đó xác minh rằng các lời gọi phụ thuộc vào executor sẽ thất bại một cách rõ ràng khi không có server, trong khi việc sử dụng có executor hỗ trợ vẫn hoạt động.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.