test: run hermetic user examples in CI

[emecii]

Which issue does this PR close?

Closes #888.

Rationale for this change

The CI workflow runs the dedicated TPC-H examples, but it does not execute the ordinary top-level user examples. These scripts are executable documentation and can drift when the public API changes.

What changes are included in this PR?

• Add examples/_tests.py, a subprocess runner for four self-contained examples: array-operations.py, create-context.py, query-pyarrow-data.py, and python-udf.py.
• Run that runner once on the ABI3 wheel after CI installs the built wheel, alongside the existing FFI and TPC-H example checks.
• Keep examples that need credentials, network access, fixture data, or optional integrations out of this first increment.

Are there any user-facing changes?

No API behavior changes. CI will now detect regressions in these four executable user examples.