chore: Update IndexedNode to always have a parentId property

[DafyddLlyr]

What?

• Adds a required parentId property to IndexedNode
• Updates sortFlow() logic to match PlanX graph structure

Why?

• Implemented in feat: Initial frontend search proof of concept with Fuse.js planx-new#3435
• For navigation to a node to work from search, we need to know the "path" through the graph to a node
• Currently sortFlow() returns an optional parentId, despite all nodes having a parent (apart from the root node). It's more accurate to give all nodes a parent. This makes things a bit easier when constructing a path (upcoming PR)
• The current implementation of sortFlow() doesn't quite match our graph structure. When reaching the next child of the root node, the current parentId is maintained instead of falling back to the root. This worked well for treating the graph as a linear structure to split it up be portal names (pseudo sections), but doesn't work in terms of actually describing the hierarchy of the graph (required to build a valid PlanX URL).
• The difference really comes down to "previous node" vs. "parent node"
• The flow will still retain the same sorted order, root.edges[0] → root.edges[1] → root.edges[2] → etc

Note: This should not impact any current functionality. sortFlow() and findNextNodeOfType() are not used anywhere in PlanX currently.

Example

• I have a root node with two edges
• I navigate through the branches of the first edges until there are no more edges
• My next step will be the second edge of the root node
• Currently: The parentId for root.edges[1] will be set to the very final node of root.edges[0] (previous node)
• This PR: The parentId for root.edges[1] will be set to "_root" (parent node)

[DafyddLlyr]

This function (and associated tests) has been removed as it's not used, and doesn't work as expected.