Make interactive visualization example optional-dependency-safe for examples verification

[Copilot]

The examples verification issue was caused by an interactive visualization example hard-failing when plotly is not installed. This made examples checks brittle in environments that intentionally omit optional visualization deps.

• Example behavior update (optional dependency handling)

  • Updated examples/visualization/example_interactive_multilayer.py to treat missing plotly as a graceful skip (exit code 0) instead of an error exit.
  • This keeps optional tooling from blocking broad example validation runs.

• Regression coverage for the specific failure mode

  • Added tests/test_example_interactive_multilayer.py to execute the example in a subprocess and assert non-failing behavior under standard example-test environment settings.

try:
    import plotly.graph_objects as go
except ImportError:
    print(" Plotly not found. Install with: pip install plotly")
    print(" Skipping interactive visualization example")
    exit(0)