refactor(ir): rename OperatorNode::map_children to with_new_children

[zzylol]

Why

OperatorNode::map_children(f) does more than map over the children: it replaces the children and rebuilds the node. Each child is replaced by f(child), then the node is rebuilt over the new children with OperatorNode::new, re-deriving its schema and result kind. Output names and qualifiers the caller overrode are kept, and guarantee and timing are cleared. The name map_children hides that rebuild, and it is shared with Operator::map_children, which is a plain map over child references (it can change the reference type, e.g. Rc<OperatorNode> → node id, and derives nothing).

This PR renames the node-level method to with_new_children, the name DataFusion uses for the same "replace the children and rebuild" operation, and says so in its doc comment.

impl OperatorNode {
    /// Replace the children and rebuild: each child is replaced by `f(child)`
    /// and the node is rebuilt over the new children, re-deriving all
    /// structural schema metadata. ...
    pub fn with_new_children(
        &self,
        f: impl FnMut(&Rc<OperatorNode>) -> Rc<OperatorNode>,
    ) -> Result<Self, SchemaDerivationError>;
}

Scope

• Renamed: OperatorNode::map_children → OperatorNode::with_new_children. Callers updated: canonicalize.rs and the schema_rebuilding / summary_coverage tests.
• Unchanged: Operator::map_children, ASAPOp::map_children, NonASAPOp::map_children (plain maps over child references, used by CSE and flattening).
• No behavior change.

Validation: workspace tests (1,625 passed), cargo fmt --check, workspace/all-target Clippy with warnings denied.

Raised in review of #646.

🤖 Generated with Claude Code