fix: make SubgraphNode.graph nullable to allow proper cleanup (#8180)

## Summary

Fix `SubgraphNode.graph` property to match `LGraphNode` lifecycle
contract. Previously declared as `override readonly graph` via
constructor parameter promotion, which prevented `LGraph.remove()` from
setting `node.graph = null`.

## Changes

- Remove `readonly` from `SubgraphNode.graph` constructor parameter
- Add `override graph: GraphOrSubgraph | null` as class property  
- Add `NullGraphError` guard in `rootGraph` getter with node ID for
debugging
- Add null guards in `ExecutableNodeDTO.resolveInput` and
`imagePreviewStore.revokeSubgraphPreviews`
- Add test verifying `rootGraph` throws after node removal

## Testing

- Existing subgraph tests pass
- New test confirms `NullGraphError` is thrown when accessing
`rootGraph` on removed node

---------

Co-authored-by: Amp <amp@ampcode.com>

src/core/graph/subgraph/proxyWidget.test.ts

@@ -31,7 +31,7 @@ function setupSubgraph(
   const subgraph = createTestSubgraph()
   const subgraphNode = createTestSubgraphNode(subgraph)
   subgraphNode._internalConfigureAfterSlots()
-  const graph = subgraphNode.graph
+  const graph = subgraphNode.graph!
   graph.add(subgraphNode)
   const innerNodes = []
   for (let i = 0; i < innerNodeCount; i++) {