refactor(GFramework.SourceGenerators): 优化ContextGetGenerator中的集合类型遍历逻辑

- 重构了EnumerateCollectionTypeCandidates方法的调用方式
- 使用模式匹配简化了类型参数的提取逻辑
- 移除了不必要的SelectMany操作提高代码可读性
- 添加了空值检查增强代码健壮性
- 在方法末尾添加了必要的换行符保持代码格式一致

GFramework.SourceGenerators/Rule/ContextGetGenerator.cs

@@ -711,11 +711,11 @@ public sealed class ContextGetGenerator : IIncrementalGenerator
         if (readOnlyList is null || fieldType is not INamedTypeSymbol targetType)
             return false;
 
-        var allTypeCandidates = EnumerateCollectionTypeCandidates(targetType)
+        foreach (var candidateType in EnumerateCollectionTypeCandidates(targetType))
-            .SelectMany(candidateType => candidateType.TypeArguments);
-
-        foreach (var candidateElementType in allTypeCandidates)
         {
+            if (candidateType is not { TypeArguments: [var candidateElementType] })
+                continue;
+
             var expectedSourceType = readOnlyList.Construct(candidateElementType);
             if (!expectedSourceType.IsAssignableTo(targetType))
                 continue;
@@ -727,6 +727,7 @@ public sealed class ContextGetGenerator : IIncrementalGenerator
         return false;
     }
 
+
     private static IEnumerable<INamedTypeSymbol> EnumerateCollectionTypeCandidates(INamedTypeSymbol typeSymbol)
     {
         yield return typeSymbol;