Reset the process-global disaggregated block cache parameters in
`PaliCallbacksTest::tearDown()` so their values do not leak across tests in the
`pali_test` binary.

1. 1. 1. Changes

• In `src/mongo/db/modules/atlas/src/disagg_storage/pali/block_cache/pali_test.cpp`,
  `PaliCallbacksTest::tearDown()` now resets `gBlockCacheSizeBytes` and
  `gBlockCacheNumShards` to `0` before calling `ServiceContextMongoDTest::tearDown()`.

1. 1. 1. Why
         The fixture tests `GetFullPageFromCache`, `GetDeltaPageFromCache`, and
         `GetMultiplePagesFromCache` set these server parameters (declared in
         `disagg_storage/server_parameters.idl`) but never restored them. Because they are
         process globals, the values persisted for the rest of the test binary. No current
         test depends on the default (`0`), so this has not caused a failure, but the
         unittest runner can randomize test order, making it a latent state leak. Resetting
         once in `tearDown()` covers all current and future fixture-based tests uniformly.