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Description

Implements IMobileObjectMetastate interface on ReadOnlyListBase to enable metastate serialization of the IsReadOnly property, completing the interface coverage across CSLA's collection types.

Changes

ReadOnlyListBase.cs

• Added IMobileObjectMetastate interface implementation with GetMetastate()/SetMetastate() methods delegating to MobileObjectMetastateHelper

MobileObjectMetastateHelper.cs

• Added ConvertJsonElement() to handle JSON deserialization returning JsonElement instead of primitive types (bool, int, etc.)
• Modified PopulateSerializationInfo() to convert JsonElement values before adding to SerializationInfo

ReadOnlyListBaseMetastateTests.cs

• Added test coverage for IsReadOnly property round-tripping through metastate serialization
• Added validation tests for null/empty metastate handling

The JsonElement conversion fix addresses a limitation where System.Text.Json deserializes object? properties as JsonElement rather than the actual primitive type, causing InvalidCastException during SerializationInfo.GetValue<T>() calls.

// Usage example
var list = dataPortal.Fetch<MyReadOnlyList>();
list.IsReadOnly = false;

// Serialize metastate
byte[] metastate = ((IMobileObjectMetastate)list).GetMetastate();

// Restore to new instance
var restored = dataPortal.Fetch<MyReadOnlyList>();
((IMobileObjectMetastate)restored).SetMetastate(metastate);
// restored.IsReadOnly now matches original value

• Fixes ReadOnlyListBase needs to implement IMobileObjectMetastate #4765

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