ci(unit-tests): stop the integration job re-running the unit tier

[JarryShaw]

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• Followed the coding style (make pylint, make mypy, make isort) -- N/A: those targets cover pcapkit/ only. Ran isort --check-only (clean) and py_compile by hand on both changed .py files.

• make test passes, and a test case covers the change -- not run in full. tests/test_tier_guard.py: 36 passed, 57 subtests. New selection, run for real in a venv built from the exact pip install -e '.[test,Scapy]' the integration/gate jobs use: 142 passed, 31 skipped, 0 failed (see caveat below on the skip count). Unit job's own selection over the 2 affected files: 131 passed, 3 skipped.

• Added a changelog entry -- N/A — changelog centralised in docs(changelog): shared 1.5.0 changelog — long-lived, merges last (#610, #616, #617, #618, #620) #657

What is the purpose of your pull request?

• ci — workflows or build tooling

Description

Part of #715. integration ran the whole suite on every matrix version despite test already running the unit tier for real on the same commit (~130 min/PR push wasted). Fix:

Job	Before	After
integration	bare pytest -q (whole suite)	positive selection via tests._tiers.fixture_tier_paths()
gate	bare pytest -q	unchanged — deliberate full-suite re-verification, not per-push, out of scope

• Selection = tests/integration + every *_runtime.py/*_regression.py file + precise node IDs for tests that only skip (not read) a generated capture in test.
• Skip-set diff (measured, not estimated): exactly 3 methods in 2 files — test_dpkt_unit.py:444,812, test_pcapng_unit.py:3058. Resolved to node IDs, not whole modules, so they don't drag in ~95 unrelated tests.
• Caveat found by cross-review, independently reproduced: 2 of those 3 node IDs are in test_dpkt_unit.py, whose classes carry @unittest.skipUnless(HAS_DPKT, ...). dpkt sits behind the separate DPKT extra, which neither integration nor gate installs (.[test,Scapy]) — true before this PR too, so not a regression, but it means only the test_pcapng_unit.py node ID actually executes for real in CI today; the 2 dpkt ones stay skipped there exactly as before, and only run for real in a fuller venv (or once/if DPKT is added to that install line, a separate, out-of-scope change). The selection is still correct and forward-compatible either way.
• Selection is asked of tests/_tiers.py at CI run time, not hand-copied into YAML — can't drift. WorkflowAgreementTests now polices this structurally; verified it fails against a deliberately regressed workflow.
• timeout-minutes: 45 untouched (CI: raise timeout-minutes on the three pytest jobs from 30 to 45 #713/ci(unit-tests): raise pytest job timeouts from 30 to 45 minutes #716's territory) — with this partition integration drops to ~2 min/version, so 45 is generous headroom, not a tight fit.

Composes with #725 (open, ci-xdist-drop-py315-matrix): both edit integration's "Run full test suite" step. On merge, keep this PR's guard-check structure and add -n auto --dist load to the final pytest -q $selection line. #725's "Report available parallelism" step is untouched and stays put.

Not closing #715 — item 1 (purge_modules to class scope) and item 2 (stop gate re-running on main pushes) remain open.

[JarryShaw]

✅ GOOD TO MERGE @ 3afdfcd7e — pending its own CI, which is still queued (0/15 required checks reported).

Cross-review ran on opus (no substitution needed) and returned GOOD TO GO. Posted here by the coordinator: that reviewer was a read-only helper with no GitHub write grant, so its verdict existed only in a hand-back report. Independently verified before posting:

Claim	Result
Merges into current main	git merge exit 0
tests/test_tier_guard.py on the merged tree	37 passed, 57 subtests
skip_idiom_test_ids()	exactly 3 node IDs, as the PR body states
dpkt behind an extra CI never installs	confirmed — DPKT extra at pyproject.toml:135; all three jobs install .[test] / .[test,Scapy]

Two corrections the reviewer asked for were applied after it reported, so its GOOD TO GO covers the pre-amend state; the delta is exactly those two fixes (corrected 161→142 passed / 31 skipped, plus the second textually-independent guard test at tests/test_tier_guard.py:290), both of which I checked directly in the diff.

Known conflict: #725 also edits this job's run step. Merge order and resolution are in the PR body.

✅ GOOD TO MERGE @ 3afdfcd7e — once CI reports.