InMemoryCacheProvider.GetOrAdd overwrites a concurrently Set value, and racing callers get an instance that isn't the cached one #85

What's wrong

InMemoryCacheProvider (Essentials.CacheProviders.InMemory/InMemoryCacheProvider.cs) doesn't declare GetOrAdd, so it inherits the default body at Essentials/ICacheProvider.cs:77-88:

if (TryGet(key, out TValue? value)) { return value!; }
TValue newValue = factory(key);
Set(key, newValue, expiration);   // unconditional cache[key] = ...
return newValue;

Check and write are separate, and Set overwrites unconditionally:

1. A concurrent Set is lost. Thread A misses and runs a slow factory; thread B calls Set(key, fresh); A's Set then replaces B's value. Deterministic repro:

   string r = cache.GetOrAdd("k", key => { cache.Set(key, "fresh-from-concurrent-writer"); return "factory"; });
   // cache now holds "factory"; the concurrent writer's value is gone

2. Racing callers diverge. Two threads calling GetOrAdd("k", _ => new object()) on a cold key (synchronised with a Barrier) each get a different instance; only the last writer's is cached. Measured: same instance: False; caller0 holds cached: False; caller1 holds cached: True.

Why it matters

The name and the ConcurrentDictionary backing both imply add-if-absent semantics: ConcurrentDictionary.GetOrAdd may run the factory twice but always returns the stored value and never overwrites. Callers using GetOrAdd to share a single instance (connections, locks, lazily built objects) silently get duplicates. #47 fixed the mirror-image race (expired-entry removal deleting a fresh Set) but did not touch GetOrAdd.

Suggested fix / acceptance criteria

• Declare GetOrAdd on InMemoryCacheProvider: run the factory, then TryAdd; if an unexpired entry exists, return it and discard the factory result; if an expired entry exists, replace only that observed entry via TryUpdate(key, newEntry, observed) with retry.
• GetOrAddAsync picks up the fix through its default.
• Tests: a Set performed inside the factory (or via the InMemoryCacheProvider.TryGet can delete a fresh value that another thread just Set, when it sees an expired entry for the same key #47-style key type) survives GetOrAdd; two concurrent cold GetOrAdd calls return the same instance, which is the cached one.
• Optionally document on ICacheProvider.GetOrAdd whether the default implementation is atomic.