feat: [Parallel] Add parallel result

[wangyb-A]

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Issue Link, if available

Description

• Removed the generic type parameter — ParallelOperation is now a concrete class returning ParallelResult
• get() now returns a ParallelResult (total, succeeded, failed branch counts + completion status) instead of null

Demo/Screenshots

Checklist

• I have filled out every section of the PR template
• I have thoroughly tested this change

Testing

Unit Tests

Have unit tests been written for these changes? Yes

Integration Tests

Have integration tests been written for these changes? Not yet

Examples

Has a new example been added for the change? (if applicable) Existing

[wangyb-A]

Next steps:

• Reuse completion config
• Integration tests
• Reevaluate early termination behaviour