feat: add VOG038 analyzer for uninitialized Value Object properties

[SteveDunn]

Implements the analyzer requested in issue #934.

Warns (VOG038) when a property whose type is a Vogen Value Object is non-nullable, lacks the 'required' modifier, and has no initializer. This pattern silently produces an invalid (uninitialized) Value Object whenever the containing type is constructed without setting the property

• a common footgun with EF Core entities and other data-transfer types.

Acceptable patterns that suppress the diagnostic:

• nullable property: public MyVo? Prop { get; set; }
• required property: public required MyVo Prop { get; set; }
• initializer: public MyVo Prop { get; set; } = MyVo.From(0);
• getter-only property: public MyVo Prop { get; } (compiler enforces init)

Changes:

• src/Vogen/Diagnostics/RuleIdentifiers.cs: add VOG038 constant
• src/Vogen/Rules/DoNotUseUninitializedValueObjectInPropertyAnalyzer.cs: new analyzer
• src/Vogen/AnalyzerReleases.Unshipped.md: register VOG038
• tests/AnalyzerTests/DoNotUseUninitializedValueObjectInPropertyTests.cs: test coverage

[JoschiZ]

I actually already wrote a fully working implementation for my own use, that I may be allowed to give you.

It handles:

• Properties
• Fields
• Assignment in the Constructor chain
• Manual auto property assignments to the backing field

[SteveDunn]

That'd be great if you could do a PR. Copilot did one for me but I haven't check it.

[JoschiZ]

Will do