Nothing in CI runs examples/*.py, and one of them is broken

Verified: the only `examples` paths in `.github/workflows/` are the five FFI crate directories and `examples/tpch`. **No top-level `examples/*.py` script is executed anywhere in CI.** Two consequences are already in the tree:

**`examples/csv-read-options.py` crashes.** It reads `data.csv` at lines 28, 39, 52, 72 and 78, and `data.csv.gz` at line 67. Neither file exists anywhere in the repository. This is not a script that runs quietly — it raises on line 28. Fix: have it write its own small CSV and gzip copy into a temporary directory at the top, which also makes it self-contained.

**Nine scripts print nothing at all.** `export.py`, `import.py`, `python-udaf.py`, `python-udf.py`, `query-pyarrow-data.py`, `sql-to-pandas.py`, `sql-using-python-udaf.py`, `sql-using-python-udf.py` and `substrait.py` contain zero `print` or `.show()` calls; they end in `assert`. They are tests wearing an example's filename. A script that prints nothing is indistinguishable to a reader from a script that does nothing. Fix is roughly two lines each: a terminal `df.show()` or `print(...)`, keeping the asserts.

**The root cause is the missing CI job**, and it is the part worth designing rather than just doing. A job running `examples/*.py` against the already-built wheel would have caught both, but it needs a skip matrix: `sql-parquet-s3.py` needs network and AWS credentials, and `sql-parquet.py`, `dataframe-parquet.py` and `sql-to-pandas.py` need an NYC taxi Parquet file the README tells you to download by hand. Worth agreeing the shape of that before writing it.

Start by reviewing .github/workflows/ and the entry points under examples/*.py, then run examples/csv-read-options.py to reproduce its missing-input failure. Define the wheel-based CI job and its skip matrix for the AWS and NYC taxi examples; done means the self-contained CSV example runs, the named silent scripts print output while retaining asserts, and eligible examples execute in CI.

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