feat: run examples/*.py in CI and fix the examples that could not run

[Developer1010x]

Which issue does this PR close?

Closes #1728.

Rationale for this change

Nothing in CI ran the top-level examples/*.py, so nothing noticed when they
stopped working. Two consequences were already in the tree, and both are the
first thing a new user copies from.

What changes are included in this PR?

examples/csv-read-options.py no longer crashes. It read data.csv and
data.csv.gz, neither of which is in the repository, so it raised on the first
read. It now writes what it needs into a temporary directory: a comma-separated
CSV, a pipe-separated and single-quoted one so the builder-pattern example
actually exercises the delimiter and quote it sets, and a gzipped one with a
comment line so the advanced-options example exercises with_comment and
with_file_compression_type. Every example now calls df.show(), so the script
demonstrates the options instead of only naming them.

Nine scripts printed nothing. 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 ended
in a bare assert. They now print their results; the asserts are unchanged.

substrait.py only worked from the repository root. It read
./testing/data/csv/aggregate_test_100.csv, so it failed from anywhere else. It
now resolves that path relative to __file__.

The CI job. A Run Python examples step, gated to abi3 like the FFI and
TPC-H steps, running from the repository root against the wheel built earlier in
the job. Placement and content are both load-bearing:

• It runs after the TPC-H generation step, because
  python-udf-comparisons.py reads examples/tpch/data/lineitem.parquet. The
  issue did not list this one; it shows up as soon as you actually run them.
• It installs pandas and polars, which are neither runtime nor dev
  dependencies but are what import.py and export.py demonstrate converting
  to and from.
• It relies on the git submodule update --init already done by the test step
  for substrait.py.
• It skips five, each for a stated reason in the workflow: sql-parquet.py,
  dataframe-parquet.py and sql-to-pandas.py need the NYC taxi Parquet file
  the README says to download by hand; ray_pickle_expr.py needs a Ray cluster;
  sql-parquet-s3.py needs network access and AWS credentials.
• A failing example does not stop the loop. All of them run and the step reports
  every failure at once, so one broken script does not hide the next.

examples/README.md gains a short "Running the examples" section with a
table of the per-example prerequisites, and drops the links to
sql-on-polars.py, sql-on-pandas.py and sql-on-cudf.py, which are not in
the repository.

Verification

All 14 non-skipped examples were run from the repository root against a
datafusion 54.0.0 wheel, with the TPC-H data generated and the testing
submodule initialised. Every one exits 0 and prints output. ruff check,
ruff format --check and codespell pass, and shellcheck is clean on the new
step's script.

One thing found on the way, not fixed here

CsvReadOptions.with_null_regex appears to have no effect at read time. With
datafusion 54.0.0:

from pathlib import Path
from datafusion import CsvReadOptions, SessionContext

p = Path("probe.csv")
p.write_text("id,name,value\n1,alice,10\n2,N/A,20\n3,carol,30\n")
ctx = SessionContext()
options = CsvReadOptions().with_has_header(True).with_null_regex(r"^(null|NULL|N/A)$")
ctx.read_csv(p, options=options).show()

N/A comes back as the literal string rather than NULL, and if the same value
sits in a column inferred as Int64 the read fails outright with
Parser error: Error while parsing value 'N/A' as type 'Int64'. The option is
plumbed through crates/core/src/options.rs into DataFusion's CsvReadOptions,
so this looks like it is below this crate. To keep this PR runnable and honest,
the advanced-options example keeps with_null_regex set and puts the N/A in a
string column. Happy to open a separate issue for it.

Are there any user-facing changes?

No API changes. The examples print their results now, and
examples/csv-read-options.py no longer requires data files that were never
shipped.

[Developer1010x]

Filed the null_regex finding mentioned above as #1735 — it turned out to be upstream in datafusion-datasource-csv, where the reader's ReaderBuilder never gets with_null_regex. Nothing in this PR depends on it.