[Bug]: FAILED invocation without ErrorObject is completed as SUCCEEDED by local emulator

### Expected Behavior

An invocation response with explicit `Status=FAILED` must result in a failed durable execution.

If an error object is required by the invocation contract, a `FAILED` response without one should be rejected or converted to a generic failure. It must never be converted into a successful execution.

### Actual Behavior

In `aws_durable_execution_sdk_python_testing.executor.Executor._validate_invocation_response_and_store()`, the `FAILED` branch passes the nullable response error to `_complete_workflow()`:

```python
case InvocationStatus.FAILED:
self._complete_workflow(
execution_arn, result=None, error=response.error
)
```

`_complete_workflow()` then chooses the terminal status based on whether the error is present:

```python
if error is not None:
self.fail_execution(execution_arn, error)
else:
self.complete_execution(execution_arn, result)
```

Consequently, this invocation response:

```json
{
"Status": "FAILED"
}
```

is recorded as `ExecutionSucceeded`.

This was observed through SAM Local with the Java SDK when `waitForCondition` exhausted its attempts inside a synchronous child context. The operation history was:

```text
StepFailed (WaitForCondition)
ContextFailed (RunInChildContext)
InvocationCompleted
ExecutionSucceeded
```

The producing Java SDK issue is aws/aws-durable-execution-sdk-java#634.

### Steps to Reproduce

The emulator can be tested directly with a handler/invoker that returns:

```python
{
"Status": "FAILED"
}
```

Alternatively:

1. Run a Java durable handler through SAM Local.
2. Inside synchronous `runInChildContext`, execute `waitForCondition` with a built-in wait strategy and a condition that always continues polling.
3. Allow the operation to exhaust its maximum attempts.
4. Inspect the durable execution history.

The step and child context are recorded as failed, but the execution is recorded as succeeded.

### SDK Version

`aws-durable-execution-sdk-python-testing` current `main` as of 2026-08-19. The affected code is used by the durable execution emulator image run by SAM Local.

### Python Version

3.13

### Is this a regression?

No known working version.

### Last Working Version

N/A

### Proposed Fix

Make the explicit invocation status authoritative:

```python
case InvocationStatus.FAILED:
error = response.error or create_generic_failure_error()
self.fail_execution(execution_arn, error)
```

Alternatively, reject a `FAILED` response whose error is missing. Do not route the `FAILED` branch through logic that infers success from `error is None`.

Add a regression test asserting that:

- `InvocationStatus.FAILED` with an error produces a failed execution;
- `InvocationStatus.FAILED` without an error also cannot produce a successful execution;
- a `ContextFailed` checkpoint does not itself force root failure when the handler legitimately catches it and returns `SUCCEEDED`.