CI: assert pyarrow is importable so Arrow/Parquet suites can't silently skip (follow-up to #236)

[ChronicallyJD]

Follow-up to #236 (the CI workflow).

Gap

The suites job installs pyarrow with pip3 install ... pyarrow, which fails the step only if pip3 itself returns nonzero. It does not catch the case that causes silent coverage loss: pip3 exits 0 but the suites' python3 cannot import the module (wrong interpreter, partial install, PATH skew). When that happens arrow_import.sh:29 sets have_pyarrow=0, the foreign-producer cross-check at :77 (and the equivalents in the other Arrow/Parquet suites) is skipped, and the suite still passes green -- the exact silent-skip the workflow argues against, one layer down.

Fix

One line after the install, so "pyarrow present" is asserted by the same interpreter the suites use and a mismatch fails the job loudly:

- run: python3 -c 'import pyarrow; print("pyarrow", pyarrow.__version__)'

Not a blocker for #236 (which is the repo's first CI and worth landing on its own); this is the follow-up so a green suites job genuinely means the Arrow/Parquet cross-checks ran.

Raised in the #236 review.

[jdatcmd]

Closed by #255 (b5ccf4b), which did more than assert it.

The assertion failed on its first run and found the root cause: python3 -c 'import pyarrow' worked as the runner and sudo python3 did not. The suites run under sudo, so root's interpreter is the one that matters, and pyarrow was landing in ~/.local. The Arrow and Parquet suites had been skipping in CI since the first green run, reporting PASS the whole time, because a skip calls pgc_summary and exits 0 — the same exit code as a pass.

Fixed by installing under sudo, and the check now asserts the import as both the runner and root, importing pyarrow.parquet rather than just pyarrow.