(PATCH): Move the Roslyn builder behind PatchClassBuilder into RoslynPatchClassBuilder

[PaulTrampert]

Stacked on #133, which adds IPatchClassBuilder. Merge that first. GitHub will then retarget this PR to main.

Why

PatchClassBuilder is both the library's public entry point and the Roslyn implementation. #95 (opt-in Emit builder) and #126 (remove the Roslyn builder) both need the public entry point to stay put while the implementation behind it changes. Splitting them now keeps those changes small. It also gives the Roslyn code its own home if it is reused later as a build-time source generator.

What changed

• New internal sealed class RoslynPatchClassBuilder : IPatchClassBuilder holds the CodeDom/Roslyn generation and its static cache, unchanged. It has a private constructor and a static Instance, like EmitPatchClassBuilder. Its body is moved verbatim from PatchClassBuilder.cs. Git shows the move as a new file rather than a rename, because PatchClassBuilder.cs still exists. Use git log -L or blame with -C on the old path for the implementation's history.
• PatchClassBuilder is now a thin facade. It keeps its type, Instance, the obsolete public constructor, and the GetPatchClassFor signature and docs. GetPatchClassFor delegates to RoslynPatchClassBuilder.Instance. Because the cache is static, instances made with the obsolete constructor still share it.
• Comments in EmitPatchClassBuilder and PatchClassModel now name RoslynPatchClassBuilder.
• Tests:
  • The shared PatchClassBuilderTest fixture runs against RoslynPatchClassBuilder.Instance.
  • The Roslyn-specific tests (concurrent first use, the public-only errors) call it directly.
  • The shared-cache test also asserts that PatchClassBuilder returns the Roslyn builder's types.

No public API changes and no behaviour changes, so this is PATCH.

Alternatives rejected

• Renaming the public class and leaving PatchClassBuilder as a subclass or alias: this would add public API for no gain, and it would tie the public type to the implementation that Remove the Roslyn-based patch class builder in favour of Reflection.Emit #126 removes.
• Changing PatchClassBuilder.Instance to return IPatchClassBuilder: this is a binary break, because the property signature changes.
• Making RoslynPatchClassBuilder public: nothing outside the library needs it, and Remove the Roslyn-based patch class builder in favour of Reflection.Emit #126 plans to remove it.
• Naming it SourceGeneratingPatchClassBuilder: the name would be confused with .NET source generators, which run at compile time, while this builder compiles at runtime.

Tests

• dotnet build: the warning set is identical to main's.
• dotnet test: core 124/124, Swashbuckle 17/17 and OpenApi 11/11 pass. The net8.0 projects were run with DOTNET_ROLL_FORWARD=Major because only the .NET 10 runtime is installed locally.

🤖 Generated with Claude Code

[github-actions]

✅ PR Title Formatted Correctly

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[PaulTrampert]

Shown as merged only because #133's branch was fast-forwarded to include this commit; nothing reached main from here. This change is now part of #133.