Implement native support for first_value window function

[weimingdiit]

Describe
Spark first_value(...) is not supported in Auron's native window execution path, so queries using it fall back instead of being executed natively.

Describe the solution you'd like
Add native support for Spark first_value(...) in Auron's window execution.

The implementation should preserve Spark semantics for:

• partitioning
• ordering
• null handling
• supported frame behavior