Add hotkey to group nodes into a node graph

[minhpham1810]

Summary

Adds the MaterialX Graph Editor Shift+C node-grouping shortcut so it actually triggers when graph nodes are selected.

When graph nodes are selected, the shortcut creates a new nodegraph, moves the selected nodes into it, and preserves relevant boundary connections so the resulting nodegraph remains usable without manually reconnecting links.

Changes

• Add Shift+C handling for grouping selected Graph Editor nodes into a nodegraph.
• Create a new nodegraph from the current selection.
• Preserve internal links between grouped nodes.
• Preserve external upstream links through nodegraph interface inputs.
• Preserve external downstream links through nodegraph outputs.
• Keep copy/cut/paste handling in the existing node editor shortcut path.
• Add a small testable shortcut predicate for grouping shortcut conditions.
• Add MaterialXTest coverage for the grouping shortcut state.

Scope Note

This PR currently includes support for grouping within nested nodegraphs, which required changes in MaterialXCore connection resolution. Based on maintainer feedback, I’m happy to narrow this PR to top-level document grouping only and move nested nodegraph support into a follow-up PR. (This feature is currently limited, now nodes can only be grouped at the top level of the document.)

Testing

• git diff --check
• MaterialXTest "Graph editor grouping shortcut"
• cmake --build build --target MaterialXGraphEditor

Related Issue

Fixes #2018

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: minhpham1810 / name: Minh Pham (07b8724, c344edb, f44b248)

[jstone-lucasfilm]

This looks very promising, thanks @minhpham1810, and take a look at the comment above for instructions on resolving the EasyCLA errors.

[minhpham1810]

@jstone-lucasfilm I just signed the CLA and changed my commit email, please have a look.

[jstone-lucasfilm]

Thanks for the excellent work here, @minhpham1810. The connection-preservation logic is more thorough than I'd expected, and the hard parts are done well.

This PR has grown into something larger than the summary suggests, though, so before we dive into code review I'd like to align on scope. Two recommended next steps would be:

1. Update the PR description to reflect the full change. It currently reads as a Shift+C shortcut fix, but it implements the whole grouping feature along with some MaterialXCore changes.
2. The key decision is nested nodegraphs. When this was first scoped, @kwokcb suggested the editor guard against nesting; this PR adds support for it instead, which is what motivates the Interface.cpp changes. I'd suggest we start with grouping at the top document level only, which would cover the original use case, keep us on well-supported ground, and require no MaterialXCore changes. We can then take up nested nodegraphs (and the resolution work they require) as a deliberate follow-up.

Once we've settled the scope, I'm glad to go deep on the grouping logic itself. Thanks again for the strong contribution!

[minhpham1810]

@jstone-lucasfilm Thank you so much for the detailed feedback! I admitted that I want a bit out of scope in this PR, so I reverted the changes I made in MaterialXCore, as well as editing the safeguard for top-level document node grouping. Please let me know if there is any other parts that I should look at.