Throw for unions mapped as entity or complex types

[Copilot]

Fixes #38376

EF doesn't support C# union types yet. Mapping one as an entity type or complex type currently produces undefined/confusing behavior instead of a clear error.

Changes

• Detection (SharedTypeExtensions): added Type.IsUnion(), which checks for System.Runtime.CompilerServices.UnionAttribute (matched by full name to avoid a compile-time dependency on the marker type).
• Validation (ModelValidator): throw CoreStrings.UnionTypeNotSupported from ValidateEntityType (union entity types) and ValidatePropertyMapping(IComplexProperty) (union complex types; also covers complex collections and value-type unions).
• Resource: new UnionTypeNotSupported string in CoreStrings.
• Tests: ModelValidatorTest cases for union entity and complex types, simulated via the real [Union] attribute.

Message

The type '{type}' cannot be mapped because union types are not supported. See #36375 for more information.

Notes

• Applies across all providers via the shared ModelValidator.
• Detection keys off the canonical [Union] marker, so it covers both class and struct unions.

[Copilot]

Pull request overview

This PR adds first-class model validation for upcoming C# union types by detecting [System.Runtime.CompilerServices.Union] / UnionAttribute on CLR types and throwing a clear, user-facing error when such types are mapped as EF entity or complex types (aligning behavior across all providers).

Changes:

• Add Type.IsUnion() detection in SharedTypeExtensions (attribute full-name match to avoid hard dependency).
• Throw CoreStrings.UnionTypeNotSupported(...) from ModelValidator when unions are encountered as entity or complex types.
• Add the new resource string and unit tests covering both entity-type and complex-type mapping scenarios.

Reviewed changes

Copilot reviewed 4 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
test/EFCore.Tests/Infrastructure/ModelValidatorTest.cs	Adds regression tests asserting unions mapped as entity/complex types produce the new clear validation error.
src/Shared/SharedTypeExtensions.cs	Introduces IsUnion() extension method based on UnionAttribute full-name detection.
src/EFCore/Infrastructure/ModelValidator.cs	Enforces the new validation rule by throwing UnionTypeNotSupported for union entity/complex types.
src/EFCore/Properties/CoreStrings.resx	Adds the user-facing error message resource for unsupported union mapping.
src/EFCore/Properties/CoreStrings.Designer.cs	Adds the generated accessor for UnionTypeNotSupported.

Files not reviewed (1)

• src/EFCore/Properties/CoreStrings.Designer.cs: Language not supported

[Copilot]

Pull request overview

Copilot reviewed 5 out of 6 changed files in this pull request and generated no new comments.

Files not reviewed (1)

• src/EFCore/Properties/CoreStrings.Designer.cs: Language not supported