.Net: Change VolatileMemoryStore.CreateCollectionAsync to not throw…

… when collection already exists (microsoft#2300)

### Motivation and Context

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today the `VolatileMemoryStore` will throw when creating a collection if
it already exists. other stores (e.g. Postgres, DuckDB) follow a
different pattern and return gracefully. throwing an error can create a
problem for parallel writes to the memory store, which is being added in
ChatCopilot [here](microsoft/chat-copilot#83).

### Description

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### Contribution Checklist


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- [x] The code builds clean without any errors or warnings
- [x] The PR follows the [SK Contribution
Guidelines](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
and the [pre-submission formatting
script](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md#development-scripts)
raises no violations
- [x] All unit tests pass, and I have added new tests where possible
- [x] I didn't break anyone 😄

dotnet/src/SemanticKernel.UnitTests/Memory/VolatileMemoryStoreTests.cs

@@ -75,17 +75,13 @@ public async Task ItCanCreateAndGetCollectionAsync()
     }
 
     [Fact]
-    public async Task ItCannotCreateDuplicateCollectionAsync()
+    public async Task ItHandlesExceptionsWhenCreatingCollectionAsync()
     {
         // Arrange
-        string collection = "test_collection" + this._collectionNum;
-        this._collectionNum++;
-
-        // Act
-        await this._db.CreateCollectionAsync(collection);
+        string? collection = null;
 
         // Assert
-        await Assert.ThrowsAsync<MemoryException>(async () => await this._db.CreateCollectionAsync(collection));
+        await Assert.ThrowsAsync<ArgumentNullException>(async () => await this._db.CreateCollectionAsync(collection!));
     }
 
     [Fact]

dotnet/src/SemanticKernel/Memory/VolatileMemoryStore.cs

@@ -22,11 +22,9 @@ public class VolatileMemoryStore : IMemoryStore
     /// <inheritdoc/>
     public Task CreateCollectionAsync(string collectionName, CancellationToken cancellationToken = default)
     {
-        if (!this._store.TryAdd(collectionName, new ConcurrentDictionary<string, MemoryRecord>()))
-        {
-            return Task.FromException(new MemoryException(MemoryException.ErrorCodes.FailedToCreateCollection, $"Could not create collection {collectionName}"));
-        }
+        Verify.NotNullOrWhiteSpace(collectionName);
 
+        this._store.TryAdd(collectionName, new ConcurrentDictionary<string, MemoryRecord>());
         return Task.CompletedTask;
     }