Fix Gradient tool inserting new nodes in the wrong position when updating a Fill node's gradient chain

[Keavon]

Fixes a bug/limitation of #4177.

[gemini-code-assist]

Code Review

This pull request introduces a new helper method insert_node_before_input to simplify and improve node splicing on wires feeding input connectors, especially for unencapsulated secondary-input branches where the upstream branch needs to be shifted to make room. A test was also added to verify this behavior. The review feedback highlights a potential issue where shared upstream nodes could be shifted multiple times due to a lack of deduplication in upstream_flow_back_from_nodes, and suggests querying node positions before mutating the network to ensure safety.

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[cubic-dev-ai]

2 issues found across 3 files

Confidence score: 3/5

• There is a concrete regression risk in editor/src/messages/portfolio/document/utility_types/network_interface.rs: insert_node_before_input may rewire a downstream input even when the inserted node lacks input 0, which can break the splice and drop the upstream connection.
• upstream_flow_back_from_nodes in editor/src/messages/portfolio/document/utility_types/network_interface.rs may emit duplicate NodeIds in shared/diamond subgraphs, which can lead to incorrect or repeated downstream processing behavior.
• The score is 3 because both findings are user-impacting graph wiring/traversal issues with moderate-to-high severity and good confidence, so this is not a low-risk merge until addressed.
• Pay close attention to editor/src/messages/portfolio/document/utility_types/network_interface.rs - node rewiring and upstream traversal may mis-handle missing input 0 and shared-path deduplication.

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