Issue Summary

While adding unit tests for the disaggregated block cache sizing parameters ([SERVER-128863]), we found that several existing tests in pali_test.cpp mutate process-global server parameters and never restore them, leaking state across tests within the unit-test binary.

Context

• Affected file: src/mongo/db/modules/atlas/src/disagg_storage/pali/block_cache/pali_test.cpp
• The PaliCallbacksTest tests GetFullPageFromCache, GetDeltaPageFromCache, and GetMultiplePagesFromCache set gBlockCacheSizeBytes (1MB) and gBlockCacheNumShards (1) but never reset them. The fixture's setUp/tearDown does not reset them either.
• These are process-global server parameters (declared in disagg_storage/server_parameters.idl), so the values persist for the remainder of the test binary after any of these tests run.
• No current test depends on the default (0) value, so this has not yet caused a failure. However, the MongoDB unittest runner can randomize test order, so a future test relying on the defaults could fail intermittently depending on ordering. This is a latent global-state leak.
• Found during review of the block cache percentage-sizing work ([SERVER-128863], epic WT-17800).

Proposed Solution

Reset the parameters once in PaliCallbacksTest::tearDown() rather than guarding each test individually:

void tearDown() override {
    // ... existing teardown ...
    // Block cache parameters are process globals; reset so they don't leak between tests.
    gBlockCacheSizeBytes = 0;
    gBlockCacheSizePct = 0;
    gBlockCacheNumShards = 0;
    ServiceContextMongoDTest::tearDown();
}

• This covers all current and future fixture-based tests uniformly and fixes the three leaking tests for free.
• Remove the now-redundant per-test parameter resets from fixture-based tests. The one non-fixture test (BlockCacheParamsTest.SizeBytesAndSizePctAreMutuallyExclusive) is not on the fixture and keeps its own ScopeGuard.

Definition of Done

• PaliCallbacksTest::tearDown() resets gBlockCacheSizeBytes, gBlockCacheSizePct, and gBlockCacheNumShards.
• Redundant per-test parameter resets removed from fixture-based tests.
• pali_test passes, including under randomized test ordering.