test: unit tests for gridUtil.list caching behaviour

Is your feature request related to a problem? Please describe. The caching layer introduced in gridUtil.list (data cache, count cache, and their interaction) has no unit test coverage. This means regressions in cache behaviour — a cache hit returning stale data, a count-cache hit incorrectly skipping the $facet branch, or a cache key collision across tenants — would not be caught before reaching production.

Describe the solution you'd like Add unit tests for gridUtil.list that cover the following scenarios:

1. Data cache hit — assert that GridModel().aggregate is not called when a valid cache entry exists for the given (tenant, filter, skip, limit, sort, cohort, detailLevel) combination.
2. Count cache hit, data cache miss — assert that the data-only pipeline (sort + skip + limit, no $facet/$count) is used and GridModel().aggregate is called exactly once without a $facet stage.
3. Cold cache (both miss) — assert that the $facet pipeline is used, and that both the data cache and count cache are populated after the call.
4. Cache key isolation by tenant — assert that a cache entry written for tenant airqo is not served to a request with a differently-cased tenant string (e.g. AirQo), and that both resolve to the same normalised key.
5. Cache key isolation by filter/sort/pagination/cohort — assert that changing any one of admin_level, skip, limit, sortBy, order, or cohort_id produces a distinct cache key and does not return a cached result from a prior request with different parameters.
6. Lazy eviction on read — assert that a stale cache entry (past its TTL) is deleted from the Map and does not prevent a fresh DB query from running.

Describe alternatives you've considered

• Relying solely on integration/E2E tests against a real MongoDB instance. Rejected because they are slower to run, harder to isolate, and would not easily let us assert internal behaviours like "aggregate was called exactly once" or "this Map entry was deleted".
• Adding the tests as part of the same PR that introduced the caching. Deferred because the PR was focused on the performance fix itself; adding meaningful cache-behaviour tests would have significantly increased its scope and review burden.

Additional context Relevant files:

• src/device-registry/utils/grid.util.js — list function (cache logic starts around the shouldCache block), _gridSummaryDataCache, _gridSummaryCountCache, makeGridSummaryDataKey, makeGridSummaryCountKey
• src/device-registry/utils/test/ut_grid.util.js — existing test file to extend

The existing tests in ut_grid.util.js already use sinon stubs for generateFilter and GridModel; the new tests should follow the same pattern. The cache Maps are module-level, so each test case should call _gridSummaryDataCache.clear() / _gridSummaryCountCache.clear() in a beforeEach hook to prevent state leaking between tests (these are not exported today — a small export or a test-only reset helper may be needed).