Implement PopulateObject for IJsonSerializer

[Copilot]

IJsonSerializer had no way to partially update an existing object from JSON — only full deserialization (replace) was available.

Changes

• IJsonSerializer — adds void PopulateObject(Stream stream, object target)
• JsonSerializer — implements it via JsonDocument + reflection: iterates JSON properties, resolves matching CLR properties case-insensitively, and deserializes each value using the serializer's configured JsonSerializerOptions; unmatched properties on the target are left untouched
• IJsonSerializerExtensions — adds PopulateObject<T>(Stream, T) and PopulateObjectFromString<T>(string, T) convenience overloads
• Public API snapshot — updated to reflect new members

Usage

var model = new MyModel { FirstName = "John", LastName = "Doe1" };

serializer.PopulateObjectFromString("""{ "lastName": "Doe2" }""", model);

// model.FirstName == "John"   (untouched)
// model.LastName  == "Doe2"   (updated)

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[GeertvanHorrik]

@copilot object initializers should be separated on new lines for readability