Add Pyomo example smoke coverage

[bernalde]

Summary

• Adds explicit Pyomo and IPOPT availability checks to examples/example_pyomo_optimizer.py.
• Makes the example return a clear nonzero failure when optional optimization dependencies are missing or when no optimization stage solves.
• Updates smoke coverage to validate missing-Pyomo, missing-IPOPT, all-stages-failed, and solved-stage success behavior.

Tests run

• python -m pytest tests/test_example_scripts.py::TestPyomoExamples -v -n 0
  • Result: passed, 5 passed in 2.75s after formatting.
• python -m ruff check tests/test_example_scripts.py examples/example_pyomo_optimizer.py
  • Result: passed.
• python -m ruff format --check tests/test_example_scripts.py examples/example_pyomo_optimizer.py
  • Result: passed, 2 files already formatted.
• python -m pytest tests/ -n auto -v -m "not notebook and not pyomo" --ignore=tests/test_pyomo_models
  • Result: passed, 247 passed in 439.92s.
• python -m mypy lyopronto/
  • Result: passed, no issues found in 12 source files. The installed mypy emitted a config warning that python_version = 3.8 is no longer supported by that mypy version, but exited successfully.

Notes

• This PR is stacked on pr/docs-examples to target the tail of PR PR 7/7: Pyomo documentation, examples, and CHANGELOG #12, where examples/example_pyomo_optimizer.py is introduced.
• Notebook tests were not run locally as part of this change because the affected behavior is in the Pyomo example script and CI runs notebooks in a separate workflow.

Closes #14

[bernalde]

Review summary:

1. Blocking issues: None.
2. Nonblocking issues: None.
3. Questions: None.
4. Tests run and outcomes:

• pytest tests/test_example_scripts.py -m "not notebook": 5 passed.
• python examples/example_pyomo_optimizer.py: passed; solved 3 of 3 optimization stages.
• python -m pytest tests/ -n auto -v -m "not notebook and not pyomo" --ignore=tests/test_pyomo_models: 247 passed, 2064 warnings.
• python -m pytest tests/test_web_interface.py -v: 8 passed.

Note: An initial broad run via the pytest console script hit local import-path failures for examples/benchmarks in this non-editable environment; rerunning the same selection through python -m pytest passed.
5. Merge as-is: Yes. GitHub would not accept an APPROVE review from this authenticated user because the user is the PR author, so I am leaving this as COMMENT.