Extension properties were not appearing in IntelliSense within object initializers, though they worked correctly for direct property access.

MyClass myClass = new MyClass()
{
    NewProperty = 3, // ❌ Missing from completion
};

myClass.NewProperty = 3; // ✅ Shows in completion

Changes

• Pass includeReducedExtensionMethods: true to LookupSymbols in AbstractObjectInitializerCompletionProvider to include extension members in the symbol lookup

Tests

Added tests in CSharpCompletionCommandHandlerTests.vb verifying:

• Extension properties appear in object initializer completion
• Read-only extension properties are excluded
• Read-only collection properties are shown for collection initializer syntax (e.g., new MyClass { ReadOnlyListProp = { ... } })
• Both regular and extension properties appear together
• Already-typed extension properties are hidden from completion

Fixes #81497
Fixes #81405

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