Generate the TestContext property for record test classes

[SimonCropp]

A record is a class, so it is a legal MSTest test class, but IsSyntaxEligibleForGeneration only matched ClassDeclarationSyntax. A [UsesVerify] [TestClass] partial record compiled and then failed at run time with the misleading:

System.Exception: TestContext is null. Ensure test class has a `[UsesVerify]` attribute (or inherits from `VerifyBase`).

pointing at an attribute that was already there.

Records are now eligible. Two things follow from that:

• A partial declaration has to repeat the kind of the type, so the emitter takes the keyword from the declaration instead of always writing partial class — which would have been a CS0261 kind mismatch against the record.
• The same applies to every enclosing type, so GetParentClasses also accepts a record struct parent. Its kind is two tokens (Keyword is only the record half, with struct in ClassOrStructKeyword), which the new GetPartialKeyword handles.

Record structs stay ineligible as targets: [UsesVerify] is declared for AttributeTargets.Assembly | AttributeTargets.Class, so putting it on one is CS0592 in the consuming source regardless of what the generator does. HasAttributeOnRecordStruct pins that as generating nothing.

Tests on both sides. Verify.MSTest.SourceGenerator.Tests gains RecordTests — record, explicit record class, record struct, and a class nested inside record → record struct parents; that harness compiles the generated output and asserts zero diagnostics, so a kind mismatch would fail it. Verify.MSTest.Tests gains an actual partial record test class as the end-to-end proof: it throws "TestContext is null" on main.

Verify.MSTest.SourceGenerator.Tests (23) and Verify.MSTest.Tests (157 across net11.0 and net48) pass.