[VL][FLINK] Avoid mutable calls to `velox4j.plan.PlanNode#setSources()`

[This API](https://github.com/bigo-sg/velox4j/blob/1239b1ea42961cacbe38294a01304f2acc0f10f3/src/main/java/io/github/zhztheplayer/velox4j/plan/PlanNode.java#L26-L28) added by `bigo-sg/velox4j` is powerful but breaks the immutability of the plan node. We look to replace the practice with one of the two possible options:

1. Refactor gluten-flink's code to avoid the `setSources` calls;
2. Add immutable `PlanNode#withNewSources` API or a builder-based API or so, which internally do copies on the query plan to change the sources.

Either way would be more elegant than the current solution.

Start with the linked PlanNode.java definition and then locate the gluten-flink code that calls setSources. Compare the two proposed approaches and determine which is accepted before changing anything. Done means mutable setSources calls are removed or replaced with an immutable source-update API, with the relevant tests passing.

Written by the indexing model from the issue text.