Add Python 3.14 to test_notebooks matrix once TensorFlow ships cp314 wheels

[skearnes]

The test_notebooks matrix in .github/workflows/run_tests.yml runs on Python 3.11–3.13 but skips 3.14, because the examples extra requires TensorFlow and no released TF version ships cp314 wheels.

As of this writing:

• TF 2.20.0 (current lock): wheels for cp310–cp313, no cp314.
• TF 2.21.0 (latest on PyPI): wheels for cp310–cp313, no cp314 — and requires protobuf>=6.31.1, which collides with our protobuf>=4.22.3,<6 pin in pyproject.toml.

When either of these unblocks (TF ships cp314 wheels, or we migrate to protobuf v6), update the test_notebooks matrix:

# .github/workflows/run_tests.yml
  test_notebooks:
    strategy:
      matrix:
        os: ["ubuntu-latest", "macos-14"]
        python-version: ["3.11", "3.12", "3.13", "3.14"]

Context: tracked from #808.