(PATCH): Drop the Roslyn patch class builder's own cache

[PaulTrampert]

RoslynPatchClassBuilder isn't used at runtime (it's kept pending #144). Now that CachingPatchClassBuilder exists (#148), the Roslyn builder no longer keeps a cache of its own.

Changes

• RoslynPatchClassBuilder: removed the static OptionalsClasses dictionary and the comments about it. GetPatchClassFor now compiles and loads a new assembly on every call, and its doc comment says so and points to CachingPatchClassBuilder. Instance stays, and its summary now says the builder holds no state, rather than mentioning a static cache.
• PatchClassBuilders (the fixture source for the shared PatchClassBuilderTest suite) wraps each builder in a CachingPatchClassBuilder. Without that, the Roslyn fixture would compile a class on every GetPatchClassFor call and the suite would slow down. The suite still tests what each builder produces, because the cache only returns what the inner builder built. The wrappers are static fields, so every time NUnit enumerates All() it gets the same two caches. The Emit builder is wrapped too, which is harmless while it still has its own cache (Route PatchClassBuilder.Instance through CachingPatchClassBuilder and drop the Emit builder's cache #149 removes that).
• RoslynPatchClassBuilderTest: removed the two cache tests (same type on repeat calls, one compile under concurrent first use). CachingPatchClassBuilderTest covers that behaviour. Added GetPatchClassFor_BuildsANewClassOnEachCall, which fails before the fix and passes after it.

ConcurrentFirstUseTestObject stays because EmitPatchClassBuilderTest still uses it. A comment in EmitPatchClassBuilder still mentions "RoslynPatchClassBuilder's cache". I left it alone so this PR doesn't touch the Emit builder, since #149 rewrites that part.

No public API change.

Test results

• dotnet build: succeeds, with 21 unique warnings both before and after this change.
• dotnet test: all passing. Core: 131. OpenApi: 12. Swashbuckle: 18.
• The new regression test fails on main, because the second call returned the same cached type.

Closes #150

🤖 Generated with Claude Code

[PaulTrampert]

@claude Since this is just a call through to the private method, we can do away with the private method.

[PaulTrampert]

Done in 77b70dd: the body of CreatePatchClass now lives in GetPatchClassFor, and the private method is gone. Build warnings unchanged; all tests pass (131/12/18).

[github-actions]

✅ PR Title Formatted Correctly

The title of this PR has been updated to match the correct format. Thank you!