OSS test fix (#436)

Summary:

Fix NvmCacheTest.EvictToNvmGet failing in OSS CI.

```
/home/runner/work/CacheLib/CacheLib/cachelib/allocator/nvmcache/tests/NvmCacheTests.cpp:252: Failure
Expected equality of these values:
  0
  nvm.getNumActiveHandles()
    Which is: 1

I0318 19:45:43.067236 184036 Driver.cpp:84] Driver: finish scheduler
I0318 19:45:43.067307 184036 Driver.cpp:86] Driver: finish scheduler successful
I0318 19:45:43.067613 184036 ThreadPoolJobScheduler.cpp:86] JobScheduler: join threads for reader_pool
I0318 19:45:43.069042 184036 ThreadPoolJobScheduler.cpp:90] JobScheduler: join threads for reader_pool successful
I0318 19:45:43.072589 184036 ThreadPoolJobScheduler.cpp:86] JobScheduler: join threads for writer_pool
I0318 19:45:43.074025 184036 ThreadPoolJobScheduler.cpp:90] JobScheduler: join threads for writer_pool successful
[  FAILED  ] NvmCacheTest.EvictToNvmGet (433 ms)
[----------] 1 test from NvmCacheTest (433 ms total)

[----------] Global test environment tear-down
[==========] 1 test from 1 test suite ran. (433 ms total)
[  PASSED  ] 0 tests.
[  FAILED  ] 1 test, listed below:
[  FAILED  ] NvmCacheTest.EvictToNvmGet

 1 FAILED TEST
```

Two bugs:

1. Unsigned underflow in loop index. The loop uses the i-- > 0 idiom which already decrements i before the body runs (values nKeys+99 down to 0). The extra index = i - 1 double-decremented, so on the last iteration (i=0), index = (unsigned)0 - 1 = UINT_MAX. This created a spurious lookup for a nonsensical key and skipped key1123. 
* **Fix**: index = i.

2. Race between async NVM callback and handle count check. When a key is fetched from NVM, the async callback runs onGetComplete →removeFromFillMap → ~GetCtx → wakeUpWaiters → baton_.post(). The baton wakes the test thread, but GetCtx::it (the fill handle) is destroyed after the baton post, still inside the same callback. Until that destruction completes, the async thread's handle count is +1. If the test thread reaches `getNumActiveHandles()` before the callback fully returns, it observes the stale +1. 

* **Fix**: call flushNvmCache() before checking handle counts. This calls scheduler_->finish() which busy-waits until all navy jobs (including the callback that destroys GetCtx::it) have fully completed.

Reviewed By: byahn0996

Differential Revision: D97184057

Author: pbhandar2

cachelib/allocator/nvmcache/tests/NvmCacheTests.cpp

@@ -215,11 +215,11 @@ TEST_F(NvmCacheTest, EvictToNvmGet) {
   const auto nEvictions = this->evictionCount() - evictBefore;
   ASSERT_LT(0, nEvictions);
 
-  // read from ram cache first so that we will not cause evictions
+  // Read from ram cache first so that we will not cause evictions
   // to navy for items that are still in ram-cache until we start
-  // reading items from navy
+  // reading items from navy.
   for (unsigned int i = nKeys + 100; i-- > 0;) {
-    unsigned int index = i - 1;
+    unsigned int index = i;
     auto key = folly::sformat("key{}", index);
     auto hdl = this->fetch(key, false /* ramOnly */);
     hdl.wait();
@@ -248,6 +248,10 @@ TEST_F(NvmCacheTest, EvictToNvmGet) {
     }
   }
 
+  // Flush to ensure all async navy callbacks (including GetCtx destruction
+  // which decrements handle counts) have fully completed.
+  nvm.flushNvmCache();
+
   // Reads are done. We should be at "0" active handle count across all threads.
   ASSERT_EQ(0, nvm.getNumActiveHandles());
   ASSERT_EQ(0, nvm.getHandleCountForThread());