[AndriiPasternak31]

Summary

Part of #3395, which #3974 fixed on dev (the issue stays open until the release). A follow-up to #3974, from @dolho's review note on MAX_BASE_WALKBACK.

When the merge-base can't build its install, scripts/ci/check_sqlite_upgrade_parity.py walks back first-parent ancestors (bounded by MAX_BASE_WALKBACK = 20) and exits 2 once the walk is exhausted. Its message told every PR to "raise MAX_BASE_WALKBACK in the fix PR". That is the right advice for the one PR that repairs the base branch. Any other PR branched inside the broken range and checked after the fix landed gets the same wrong advice: its merge-base is still the broken commit, and what it needs is to merge the repaired base branch in, which moves the merge-base past the break with no change to the bound.

Changes

• The exhausted-walk message now names both cases.
• The comment on MAX_BASE_WALKBACK says only the fix PR can ever need a larger bound. The bound stays at 20: schema-parity has about 200 push runs on dev since mid-July with none failing, and 20 first-parent commits is roughly a day and a half of dev.
• New self-test test_exhausted_walk_clears_once_the_repaired_base_is_merged_in: a PR branched inside the broken range, with the bound at 1, exits 2 and is told to merge the base branch in; after merging it, the guard passes from the fixed merge-base with no walk-back warning.
• tests/registry.json: one sentence added to the existing entry's description.

No docs: CI-script message change (bug-fix tier).

Journey Impact: none: a CI guard's failure message and its self-test. No runtime code, endpoint, UI or migration behaviour changes.

Test Plan

Mutation: dev's scripts/ci/check_sqlite_upgrade_parity.py restored with the tests kept → tests/unit/test_sqlite_upgrade_parity_guard.py 1 failed, 26 passed (test_exhausted_walk_clears_once_the_repaired_base_is_merged_in, on the advice assertion).

Validation: tests/unit/test_sqlite_upgrade_parity_guard.py → 27 passed.

• test_sqlite_upgrade_parity_guard.py test_schema_parity.py test_migrations.py test_alembic_parity_guard.py test_2338_journey_catalog.py → 131 passed under pytest-randomly seeds 11, 222 and 3333 (Python 3.12).
• ruff clean on both files; mypy --strict clean on the script; tests/lint_sys_modules.py and tests/lint_root_test_placement.py pass; git diff --check clean; gitleaks over the branch commit found no leaks.

Unverified: not run under CI's Python 3.13 image (the change is a string and a test over synthetic git repositories); the full tests/unit tier was not run.

[vybe]

merge-train: batch validated on train/20261028-0026 (train PR #4045 green: regression diff, journey-smoke, prod-image-smoke, pg-migrations, schema-parity, CodeQL, verify-non-root). Validated head 36c7098.