fix: Removing node with edge instances

[hoxbro]

Description

Fixes this raising an AttributeError when deleting Node A.

import panel as pn
import panel_reactflow as pr

# pn.extension("jsoneditor")

nodes = [
    pr.Node(id="a", position={"x": 0, "y": 0}, label="Node A"),
    pr.Node(id="b", position={"x": 250, "y": 0}, label="Node B"),
    pr.Node(id="c", position={"x": 125, "y": 150}, label="Node C"),
]

edges = [
    pr.Edge(id="e1", source="a", target="c"),
    pr.Edge(id="e2", source="b", target="c"),
]

flow = pr.ReactFlow(nodes=nodes, edges=edges, sizing_mode="stretch_both")

pn.Column(flow, sizing_mode="stretch_both").servable()

How Has This Been Tested?

AI Disclosure

• This PR contains AI-generated content.
  • I have tested all AI-generated content in my PR.
  • I take responsibility for all AI-generated content in my PR.

Tools and Models: Cursor + Sonnet 4.6

Checklist

• Tests added and are passing