Add copy/paste integration tests

[coderabbitai]

Summary

Add comprehensive copy/paste integration (e2e) tests to improve coverage of paste behavior for all graph item types.

Background

PR #9962 fixed the paste position for LGraphGroup instances and added a basic e2e test (browser_tests/tests/groupCopyPaste.spec.ts) scoped to that specific fix. However, broader copy/paste integration tests are still needed to cover the full range of copy/paste scenarios.

Suggested Test Cases

• Copy/paste of LGraphNode items — verify they paste at cursor offset
• Copy/paste of Reroute items — verify they paste at cursor offset
• Copy/paste of LGraphGroup items — verify they paste at cursor offset (covered by fix: LGraphGroup paste position #9962)
• Copy/paste of mixed selections (nodes + groups + reroutes)
• Edge cases: pasting with no selection, pasting multiple times, etc.

References

• PR: fix: LGraphGroup paste position #9962
• Comment: fix: LGraphGroup paste position #9962 (review)

Requested by @DrJKL

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