Add hotkey to group nodes into a node graph

[minhpham1810]

Summary

Moves the Graph Editor's node-grouping logic into a reusable, unit-tested core method, per maintainer feedback. The editor now delegates to core, grouping stays top-level-only, and the Shift+C hotkey remains a thin trigger.

Changes

• source/MaterialXCore/Node.h — Declare new GraphElement::createNodeGraphFromNodes(nodes, name), which groups the given nodes into a new child nodegraph while preserving internal and boundary connections.
• source/MaterialXCore/Node.cpp — Implement createNodeGraphFromNodes plus file-local helpers (ported from the editor) that copy the selected nodes into a new nodegraph, turn external upstream links into nodegraph interface inputs, rewire external downstream consumers to nodegraph outputs (with fan-out dedup), and remove the originals. The method is general and does not set layout attributes or enforce a document-only policy.
• source/MaterialXGraphEditor/Graph.cpp — Remove the in-editor grouping helpers and rewrite groupSelectedNodesIntoNodeGraph to collect the selected nodes and delegate to the core method, keeping only editor concerns (top-level guard, average node position, graph rebuild/relink/render).
• source/MaterialXTest/MaterialXCore/Node.cpp — Add [nodegraph] unit tests covering internal links, upstream/downstream boundary rewiring, fan-out output dedup, original removal, document validation, and the empty-selection case.
• source/MaterialXCore/Interface.cpp — Remove a redundant duplicate resolveNameReference<NodeGraph> no-op in PortElement::validate left over from the reverted nesting rework.

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.

[kwokcb]

I would agree that to only allow top level / document level grouping to occur.

( For reference, I did rework to support graph nesting and fix a other related issues in this PR . )

BTW, there was another PR proposal which took advantage of the cut/paste mechanism.

From a cursory look and revisit of the previous PR, my suggested approach is to

1. Put new "grouping" functionality in core so it's a reusable and testable function.
2. Then expose it in editor.
3. Then add the hotkey as a minor addition.

[minhpham1810]

Thank you for the constructive review and feedback. From what we've been discussing so far, I will proceed with what @kwokcb suggested in the comment above. Please let me know if there is any further instructions or directions that you want to highlight @kwokcb @jstone-lucasfilm .

[minhpham1810]

@jstone-lucasfilm @kwokcb I just push the changes that relocate grouping logic into core. Please have a look.