(PATCH): Extract the patch class model from PatchClassBuilder

[PaulTrampert]

Closes #127

What and why

Groundwork for the Reflection.Emit builder (#95). The decisions about what a patch class contains move out of PatchClassBuilder.CreatePatchClass into a new internal PatchClassModel, so a second builder can share them instead of copying them and drifting (the bug class behind #87–#93). Doing the move here keeps the Emit PR's diff to new IL only.

PatchClassModel.For(type) holds:

• IsRecord (the <Clone>$ check),
• Constructor and ConstructorProperties (System.Text.Json's constructor selection and case-insensitive parameter binding, in parameter order),
• OptionalProperties and IgnoredProperties (most-derived properties with a public setter or init accessor, or bound by the constructor, split on [JsonIgnore(Condition = Always)]),
• for each optional property, an OptionalPropertyModel with HasConverter, JsonPropertyName, and Validators (OptionalValidatorModel(type, index), with the index counted per validator type).

PatchClassBuilder turns the model into CodeDom/C# exactly as before. The helper methods moved verbatim (IsRecord became DetectRecord so it doesn't clash with the property). Some things stayed in the builder on purpose, because they're about generating the class, not about what it contains:

• the IsVisible check and its NotSupportedException, which is a Roslyn restriction. It still runs before the model is built, so the order of exceptions is unchanged;
• leaving ignored properties out of the with expression for records;
• naming and escaping.

No public API changes. The model types are internal, and the core project gains [InternalsVisibleTo("PTrampert.SimplePatch.Test")] (as an InternalsVisibleTo item in the csproj).

Alternatives rejected

• Have the model also say what goes in the constructor and what goes in the initializer for records. That's a choice about how to build the instance, and an Emit builder may make it differently, so it stays in the builder.
• Keep the JsonConverterAttribute instance in the model instead of a HasConverter flag. The generated attribute only ever needs the source type and property name, so the flag is all a builder needs.

Tests

• PatchClassBuilderTest passes unchanged.
• New PatchClassModelTest (10 tests) covers the model directly: positional records, constructor-bound get-only properties (including a [JsonIgnore] one), a get-only property not in the constructor, [JsonConstructor], ambiguous constructors, hidden properties, JsonIgnore conditions, carried attributes, and repeated validators of one type. This is a pure refactor, so these tests pin the behaviour rather than reproduce a bug. They don't compile before the change because the model didn't exist yet.

Results:

• dotnet build: 0 errors, 21 warnings, the same as main.
• dotnet test: SimplePatch.Test 86/86, Swashbuckle.Test 17/17, OpenApi.Test 11/11. The machine has no .NET 8 runtime, so the net8.0 test projects ran with DOTNET_ROLL_FORWARD=Major.

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