refactor: InMemoryProvider throwing when types mismatched (open-feature#442)

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## This PR
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- Update InMemoryProvider to return an ErrorType with a default value
instead of throwing exceptions
- Add unit test to cover the new behavior

### Related Issues
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Fixes open-feature#441

### Notes
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### Follow-up Tasks
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### How to test
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Signed-off-by: Kyle Julian <38759683+kylejuliandev@users.noreply.github.com>
Signed-off-by: Weihan Li <weihanli@outlook.com>

Author: kylejuliandev

src/OpenFeature/Providers/Memory/InMemoryProvider.cs

@@ -3,7 +3,6 @@
 using System.Threading;
 using System.Threading.Tasks;
 using OpenFeature.Constant;
-using OpenFeature.Error;
 using OpenFeature.Model;
 
 namespace OpenFeature.Providers.Memory
@@ -112,7 +111,7 @@ private ResolutionDetails<T> Resolve<T>(string flagKey, T defaultValue, Evaluati
                 return value.Evaluate(flagKey, defaultValue, context);
             }
 
-            throw new TypeMismatchException($"flag {flagKey} is not of type {typeof(T)}");
+            return new ResolutionDetails<T>(flagKey, defaultValue, ErrorType.TypeMismatch, Reason.Error);
         }
     }
 }

test/OpenFeature.Tests/Providers/Memory/InMemoryProviderTests.cs

@@ -186,9 +186,14 @@ public async Task MissingFlag_ShouldReturnFlagNotFoundEvaluationFlag()
         }
 
         [Fact]
-        public async Task MismatchedFlag_ShouldThrow()
+        public async Task MismatchedFlag_ShouldReturnTypeMismatchError()
         {
-            await Assert.ThrowsAsync<TypeMismatchException>(() => this.commonProvider.ResolveStringValueAsync("boolean-flag", "nope", EvaluationContext.Empty));
+            // Act
+            var result = await this.commonProvider.ResolveStringValueAsync("boolean-flag", "nope", EvaluationContext.Empty);
+
+            // Assert
+            Assert.Equal(Reason.Error, result.Reason);
+            Assert.Equal(ErrorType.TypeMismatch, result.ErrorType);
         }
 
         [Fact]