fix(OMN-16709): restore sys.modules in the scaffold fixture so the unrestored-del ratchet passes

[jonahgabriel]

OMN-16709 — unblocks every omnibase_core PR on dev

CI Summary is a required status check on omnibase_core dev and fail-closes behind
Tests Gate. Tests Gate has been red on every branch cut from dev since #1598
(OMN-16679) merged at 2026-08-27T00:00:29Z (240ca9f205c35274094dff7ca133f576b1ef17db),
because the failing test is a static AST scan of the whole tests/ tree — it does not care
what the PR under test changed.

tests/unit/test_omn_14944_sys_modules_purge_regression.py::test_no_unrestored_del_sys_modules_in_tests
AssertionError: Unrestored `del sys.modules[...]` found (OMN-14944 contaminator class -- see OMN-14985).
assert not ['tests/unit/cli/test_cli_new_scaffold_runnable.py:50']

Net effect: no omnibase_core PR could merge to dev, regardless of content. Observed live on
the docs-only #1600 (OMN-16581), reproduced byte-identically across jobs 98394415201 and
98400912799 — deterministic, not flaky.

Root cause

The importable_src fixture added by #1598 ends with a bare del sys.modules[name] loop. The
OMN-14985 ratchet (_function_restores_sys_modules) only accepts a del sys.modules[...] when
the enclosing function also contains a with isolated_sys_modules(...) block, a
sys.modules[key] = ... assignment, or a sys.modules.update(...) call. The fixture has none
of the three.

This was a false positive — and the obvious fix is the wrong one

The OMN-14944 leak class is: evict pre-existing omnibase_core.* entries and never restore
them. This fixture does the opposite — it deletes only set(sys.modules) - original_modules,
i.e. entries that did not exist before the test. That is cleanup of the scaffolded temp
package, which genuinely must not leak across xdist-shared workers.

isolated_sys_modules was deliberately not used: it has different semantics (it evicts
pre-existing entries and restores them on exit, while explicitly leaving in place entries
newly imported inside the block). Swapping it in would have satisfied the guard while silently
deleting the scaffold-package purge this fixture exists to perform, reintroducing the very
cross-worker leak #1598 was guarding against.

Fix

Snapshot sys.modules as a dict, keep the newly-added-name purge exactly as-is, then restore
the pre-existing entries explicitly:

original_modules = dict(sys.modules)
...
for name in set(sys.modules) - set(original_modules):
    del sys.modules[name]
sys.modules.update(original_modules)

Strictly stronger than before: the purge is unchanged, and any pre-existing entry a scaffold
import displaced is now also restored. It satisfies the ratchet through the
sys.modules.update(...) branch as an honest restore, not a guard-appeasement no-op.

One file, +9/-2, tests only. No source change.

dod_evidence

• tests/unit/test_omn_14944_sys_modules_purge_regression.py — 3 passed; repo-wide
  violations list empty.
• tests/unit/cli/test_cli_new_scaffold_runnable.py — 8 passed (the OMN-16679 acceptance
  tests still pass, so the fixture's purge behavior is unchanged).
• Combined local run on the pushing host: 11 passed in 5.53s.
• Governed pre-push selector ran the impacted subset with no overrides — verified
  is_full_suite=False, full_suite_reason=None, 44 selected paths, and no bare tests/ or
  tests/unit/ entry (so the OMN-15408 whole-suite-equivalent guard correctly did not fire).
  env | grep -Ei 'PREPUSH|ENABLE_SMART_TESTS' → none present. No --no-verify, no
  PREPUSH_FULL_SUITE, no ENABLE_SMART_TESTS, no PREPUSH_ALLOW_*.
• Final proof is this PR's own green Tests Gate + CI Summary, plus previously-blocked docs(OMN-16581): close Wave-3 audit residuals A+B — Makefile ci.yml rename leftover + stale omnidash handshake template #1600
  (OMN-16581) going green on a re-run with no change to its own diff.

Fixes OMN-16709

Evidence-Ticket: OMN-16709
Evidence-Source: OCC#7260

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• tests/unit/cli/test_cli_new_scaffold_runnable.py

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