SPOT : UserDataStreamEventsResponse does not deserialize correctly, instance is always None

### Description

The `UserDataStreamEventsResponse` model fails to parse user data stream events into their concrete event types (`BalanceUpdate`, `ExecutionReport`, `OutboundAccountPosition`, `EventStreamTerminated`, etc.). All input results in `None` and the event data is completely lost.

### Expected Behavior

When validating a user data stream event, the model should:
1. Parse the event dictionary into the appropriate concrete event type based on the `"e"` field
2. Return a strongly-typed instance (e.g., `BalanceUpdate`, `ExecutionReport`)
3. Preserve all event data fields

Example:
```python
from binance_sdk_spot.websocket_api.models import UserDataStreamEventsResponse

event = {
"e": "balanceUpdate",
"E": 1573200697110,
"a": "BTC",
"d": "100.00000000",
"T": 1573200697068
}

result = UserDataStreamEventsResponse.model_validate(event)
# Expected: result is a BalanceUpdate instance with all fields accessible
# Expected: result.a == "BTC"
# Expected: result.d == "100.00000000"
# Expected: result.E == 1573200697110
# Expected: result.e == "balanceUpdate"
# Expected: result.T == 1573200697068
```

### Actual Behavior

The parsing always fails and returns `None`, losing all event data:

```python
result = UserDataStreamEventsResponse.model_validate(event)
# Actual: result.actual_instance is None
# Actual: All event data is lost
# Actual: Cannot access event fields
```

### Possible Root Cause

The model uses Pydantic's discriminated union pattern but fails to properly discriminate between event types. Issues include:

1. **Schema Validation Failure**: The discriminator field or schema mapping is not correctly configured
2. **Type Resolution Failure**: The model cannot resolve which concrete event type to instantiate
3. **Silent Failure**: Instead of raising validation errors, it silently returns `None`

This is similar to the `SymbolFilters` discriminated union issue that was previously identified, where filter type discrimination failed.

### Impact

- **Complete Data Loss**: All user data stream events are lost when using this model
- **No Balance Updates**: Balance changes from `outboundAccountPosition` events cannot be processed
- **No Order Updates**: Order status changes from `executionReport` events cannot be tracked
- **No Stream Monitoring**: Stream termination events cannot be detected

### Related Issues

- Similar to #476 discriminated union parsing issue

### References

**Affected Version:** >=2.1.0
Related or same as #405

Beginne mit UserDataStreamEventsResponse.model_validate und den zugehörigen konkreten Ereignismodellen und vergleiche anschließend das Verhalten der diskriminierten Union mit dem SymbolFilters-Problem in #476 und dem zugehörigen Problem #405. Als erledigt gilt die Aufgabe, wenn ein Ereignis wie balanceUpdate eine typisierte Ereignisinstanz erzeugt und seine Felder beibehält, anstatt actual_instance als None zu belassen.

Vom Indexierungsmodell aus dem Issue-Text verfasst.