Node tools used to create `ExhibitStateUsage` should not be available on the background of all the types of diagram

[gcoutable]

Steps to reproduce

1. Create a SysON project with an Action Flow View diagram.
2. Display the background palette

Expected behavior

Tools New Exhibit State and New Exhibit Parallel State should not be available using the diagram palette search tool.

Actual behavior

Tools New Exhibit State and New Exhibit Parallel State are available in the diagram palette.

[gcoutable]

It occurs because SDVDiagramDescriptionProvider#addCustomTools is called without a guard, making all custom tools—specifically New Exhibit State and New Exhibit Parallel State—available across all diagrams.

Update ExhibitStateNodeToolProvider to create a diagram palette NodeTool that includes a precondition expression calling DiagramQueryToolService#toolShouldBeAvailable should address the issue.
Meanwhile, the node palette NodeTool for ExhibitStateUsage should remain without a precondition expression.