Add copy/paste integration tests

## 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 #9962)
- Copy/paste of mixed selections (nodes + groups + reroutes)
- Edge cases: pasting with no selection, pasting multiple times, etc.

## References

- PR: https://github.com/Comfy-Org/ComfyUI_frontend/pull/9962
- Comment: https://github.com/Comfy-Org/ComfyUI_frontend/pull/9962#pullrequestreview-2712000000

Requested by @DrJKL

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