coverage: install lz4 for the unit-test step

[roydahan]

Update: #1088 ("connection: don't auto-negotiate LZ4 from the built-in codec alone") has merged, so this PR no longer waits on anything. Master, including #1115, has been merged into this branch. The unit-test step now reads uv run --with lz4 --with "pyasyncore; python_version >= '3.12'" coverage run ..., and the comment blocks for both overlays are kept.

Summary

The coverage lane builds with CASS_DRIVER_NO_CYTHON=1, so the Cython-only cassandra.cython_lz4 codec isn't built. The lz4 package is only available through the optional compress-lz4 extra. As a result, the lane has no lz4 codec at all:

• the lz4 import/compat block in cassandra/connection.py (lines 72-101) never runs;
• 7 lz4-dependent tests in tests/unit/test_segment.py are skipped.

This PR adds --with lz4 to the main unit-test uv run coverage run call in scripts/coverage.sh. uv run --with layers lz4 on top of the project env for that one call only, and leaves the project env itself unchanged, so later steps run without lz4. Coverage still reads the same [tool.coverage] config and writes parallel data files, which coverage combine merges as before.

Not changed: the workflow, the dev group, pyproject.toml, the asyncio reactor step, and the integration step.

Why only the unit step

Cluster.compression defaults to True, and the connection picks the first codec that both the driver and the server support. If lz4 were installed for the whole lane, the ~20-minute integration run would switch to LZ4.

With #1088, compression=True picks LZ4 only when the lz4 package is installed; otherwise the connection stays uncompressed. That makes LZ4 opt-in, so the integration step should keep measuring the default, uncompressed path that most users get.

Unit tests are the right place to cover the lz4 code, including the "lz4 package installed" branch that #1088 adds. The asyncio reactor step doesn't exercise compression (its helpers send 'COMPRESSION': []), so it doesn't get lz4 either.

Before / after

Unit-only scripts/coverage.sh on Python 3.13, no SCYLLA_VERSION, measured before the merges described above:

	master (c362d53)	this PR
test_segment.py skipped	7	0
unit run	1462 passed, 59 skipped	1469 passed, 52 skipped
cassandra/connection.py	68% (403 missed, incl. 72-101)	68% (392 missed; only fallback lines 74-75 and 80-81 of that block remain)
cassandra/segment.py	85% (16 missed)	98% (3 missed)
TOTAL	63% (6655 missed)	63% (6630 missed)

In CI (unit + integration), the first run of this PR measured 3232 missed statements, against 3278 on master at the time.

The 7 tests in tests/unit/cython/test_cython_lz4.py still skip, as expected, because the Cython codec isn't built in this lane.

Verification

• Ran the unit-only scripts/coverage.sh on master and on this branch; numbers are in the table above. Both runs exited 0, and the asyncio reactor step passed 2/2 in both.
• After the branch run, import lz4 fails in the project env, so lz4 isn't left behind and the integration step runs without it.
• After merging master: bash -n passes. lz4 and asyncore (via pyasyncore) both import inside the combined --with overlay. test_segment.py and test_asyncorereactor.py pass together under it (31 passed). CI on the merged head is green.
• The integration-step command is unchanged (see diff).
• Not verified locally: the integration step itself, since no cluster was available. This PR's coverage job runs it.

Pre-review checklist

• I have split my patch into logically separate commits.
• All commit messages clearly explain what they change and why.
• I added relevant tests for new features and bug fixes. (N/A: CI script change; it un-skips existing tests)
• All commits compile, pass static checks and pass test. (unit suite; integration via this PR's CI)
• PR description sums up the changes and reasons why they should be introduced.
• I have provided docstrings for the public items that I want to introduce. (N/A)
• I have adjusted the documentation in ./docs/source/. (N/A; CONTRIBUTING.rst checked and still accurate)
• I added appropriate Fixes: annotations to PR description. (N/A)

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ℹ️ Recent review info

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📥 Commits

Reviewing files that changed from the base of the PR and between fc285ca and 4a03231.

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📒 Files selected for processing (1)

• scripts/coverage.sh

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📝 Walkthrough

📝 Walkthrough

Walkthrough

The unit-test coverage command adds lz4 to its temporary environment. The conditional pyasyncore dependency remains unchanged. The integration coverage command does not add lz4.

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Priority: ⬇️ Low

Change: Other

Merge Risk: ⚪ Minimal · up to 4a032

This change makes LZ4 available only to the unit-test run; the integration run retains its uncompressed path. No merge-blocking risk is established.

Pre-merge checks | 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Docstring Coverage		No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 1…
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.
Title check		The title clearly and concisely identifies the main change: installing lz4 for the unit-test coverage step.
Description check		The description follows the template, explains the change and rationale, records verification results, and marks non-applicable checklist items. It also identifies that the integration step was not ve…

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