Refactoring QueryPlanSerde in Apache DataFusion-Comet

PRs: #2028 and #2085 — part of tracking issue #2019 Background DataFusion-Comet translates Spark physical plans into DataFusion execution plans. The core of this translation lives in QueryPlanSerde, a Scala file responsible for serializing Spark expressions into Protocol Buffer messages. Over time, QueryPlanSerde had accumulated serialization logic for every expression type — comparisons, datetime operations, string functions, math — all in a single file. This made navigation difficult, PR reviews cumbersome, and adding new expressions error-prone.