test(autoCombo): port models_dev_tier #11508 guard from node:test to vitest

[jonlwheat2-gif]

Fixes the Vitest (fast-path) base-red on release/v3.8.51 (tracked under #11449; surfaced on PR #11633's run).

Root cause

tests/unit/autoCombo/models-dev-tier-11508.test.ts imports describe / it / before from node:test, but the directory is collected exclusively by vitest:

• vitest.mcp.config.ts includes "tests/unit/autoCombo/**/*.test.ts".
• The Node-native runner (test:unit) only globs tests/unit/*.test.ts, tests/unit/dashboard/**, and tests/unit/**/*.test.mjs — it never sees this file.
• Every sibling in tests/unit/autoCombo/ imports from "vitest".

Result: vitest collects the file, registers zero suites, and fails the whole job with No test suite found in file ... models-dev-tier-11508.test.ts, while the node:test callbacks leak their own spec output into the vitest log. The suite has effectively never been counted by either runner.

Fix

Port the imports to vitest — before → beforeAll; keep node:assert/strict (runner-agnostic). No assertion or seeding logic changed. 2-line diff.

Verification

# pristine origin/release/v3.8.51 (broken):
Test Files  1 failed | 48 passed (49)
Tests       452 passed

# this branch:
Test Files  49 passed (49)
Tests       457 passed   # +5: this file's tests now actually register

Note: the other failing fast-path job on that run (Docs Gates — provider-count 353→354 drift) is a separate concern and needs its own docs-sync fix.