No ExclusiveJoinTask builder class — defaultExclusiveJoinTask field unreachable

## Summary

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

`TaskType.EXCLUSIVE_JOIN` exists in the SDK enum but there is no `ExclusiveJoinTask`
builder class. More critically, the `defaultExclusiveJoinTask` field (which tells
the server which task to use when no SWITCH branch ran) cannot be set through any
SDK path.

## Details

`JoinTask` always produces `TaskType.JOIN`, not `TaskType.EXCLUSIVE_JOIN`. There
is no way to build an EXCLUSIVE_JOIN task using SDK classes alone.

EXCLUSIVE_JOIN is designed for SWITCH branches (where exactly one branch executes).
Without `defaultExclusiveJoinTask` set, EXCLUSIVE_JOIN workflows with a branch that
doesn't match will stall.

## Suggested fix

```python
class ExclusiveJoinTask(TaskInterface):
def __init__(self, task_ref_name: str, join_on: List[str],
default_exclusive_join_task: Optional[List[str]] = None):
super().__init__(task_ref_name, TaskType.EXCLUSIVE_JOIN)
self._join_on = join_on
self._default = default_exclusive_join_task or []

def to_workflow_task(self) -> WorkflowTask:
wf = super().to_workflow_task()
wf.join_on = self._join_on
wf.default_exclusive_join_task = self._default
return wf
```

## Verified against

Conductor server **3.32.0-rc.9**. EXCLUSIVE_JOIN runs correctly when built via
raw `WorkflowTask` with the correct fields set. The gap is purely in the SDK —
no builder class exists.

Locate the existing JoinTask and TaskInterface entry points, then inspect how they create a WorkflowTask and assign TaskType. Add coverage for constructing an EXCLUSIVE_JOIN task with join_on and default_exclusive_join_task; done means the SDK can serialize those fields and the resulting workflow runs correctly for an unmatched SWITCH branch.

由索引模型根据 Issue 内容生成。