Reevaluate CompletionConfig Presets

## Context
**GitHub Reference**: [config.py#L47](https://github.com/aws/aws-durable-execution-sdk-python/blob/0307cf96e22a0bdbfdb7539f2780cde19f44a55d/src/aws_durable_execution_sdk_python/config.py#L47)

Review and update `CompletionConfig` presets for `first_successful`, `all_completed`, and `all_successful` to ensure they match expected behavior and service capabilities.

This is currently not implemented in TypeScript.

## Technical Requirements
- Review current preset implementations
- Validate against service behavior
- Update presets if needed
- Ensure consistency with TypeScript SDK

## Implementation Details
- **File**: `src/aws_durable_execution_sdk_python/config.py`
- **Line**: Around line 47
- **Presets to Review**:
- `first_successful`: Complete when first operation succeeds
- `all_completed`: Complete when all operations finish (success or failure)
- `all_successful`: Complete when all operations succeed

```python
@dataclass(frozen=True)
class CompletionConfig:
@classmethod
def first_successful(cls) -> CompletionConfig:
"""Complete when first operation succeeds."""
return cls(
completion_type="FIRST_SUCCESSFUL",
min_successful=1,
fail_on_first_error=False
)

@classmethod
def all_completed(cls) -> CompletionConfig:
"""Complete when all operations finish."""
return cls(
completion_type="ALL_COMPLETED",
min_successful=None,
fail_on_first_error=False
)
```

## Acceptance Criteria
- [ ] All presets match expected behavior
- [ ] Presets are consistent with service capabilities
- [ ] Documentation clearly explains each preset
- [ ] Tests validate preset behavior
- [ ] Alignment with TypeScript SDK confirmed

**Priority**: LOW
**Estimated Effort**: 0.5 days

从 src/aws_durable_execution_sdk_python/config.py 第 47 行附近开始，检查 first_successful、all_completed 和 all_successful 这些 preset。将其文档中描述的行为与服务能力和 TypeScript SDK 进行比较，然后添加或更新文档和测试，以验证每个 preset 的行为并满足验收标准。

由索引模型根据 Issue 内容生成。