Use generic interface for node types

[marko1olo]

Replaced hard-coded node types with a generic IGeneratesVirtualTextureFeedback interface in VirtualTexturingFeedbackUtils.

This patch:

• Creates IGeneratesVirtualTextureFeedback interface to unify how virtual texturing node classes provide feedback variables.
• Modifies SampleVirtualTextureNode and SubGraphNode to implement the interface.
• Adapts VirtualTexturingFeedbackUtils.cs to query and use the interface rather than performing explicit types checks, resolving technical debt and increasing system extensibility.
• Updates generic constraints in GraphUtil.cs (FindDownStreamNodesOfType) to support class (including interfaces).

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PR created automatically by Jules for task 11557214558165152221 started by @marko1olo

[google-labs-jules]

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[marko1olo]

✅ Confirmed Integrated: Code is present in main branch (Direct commit match in git log). Closing PR.

[google-labs-jules]

✅ Confirmed Integrated: Code is present in main branch (Direct commit match in git log). Closing PR.

Understood. Acknowledging that this work is now obsolete and stopping work on this task.