Emit all properties on the join type, even when there's no extra configuration

[AndriySvyryd]

Fixes #35524

When scaffolding a many-to-many relationship where the join table has a composite foreign key with 3 or more properties, the generated OnModelCreating code can omit one of them if it had no extra fluent API configuration.

[Copilot]

Pull request overview

This pull request fixes issue #35524 where scaffolding a many-to-many relationship with a composite foreign key containing 3 or more properties would omit properties from the generated OnModelCreating code if they had no extra fluent API configuration. This resulted in runtime errors due to incompatible foreign key types.

Changes:

• Modified the T4 template logic in CSharpDbContextGenerator.tt to emit IndexerProperty calls for all properties in join entity types, regardless of whether they have additional fluent API configuration
• Added comprehensive test coverage for the scenario with composite foreign keys in many-to-many relationships
• Updated existing test baselines to reflect that all join entity properties are now emitted

Reviewed changes

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

File	Description
src/EFCore.Design/Scaffolding/Internal/CSharpDbContextGenerator.tt	Fixed the logic to emit IndexerProperty for all join entity properties, not just those with extra configuration
src/EFCore.Design/Scaffolding/Internal/CSharpDbContextGenerator.cs	Generated C# code from the T4 template reflecting the fix
test/EFCore.Design.Tests/Scaffolding/Internal/RelationalScaffoldingModelFactoryTest.cs	Added test to verify model scaffolding with composite foreign keys in many-to-many relationships
test/EFCore.Design.Tests/Scaffolding/Internal/CSharpEntityTypeGeneratorTest.cs	Added comprehensive code generation test and updated baselines for existing tests to include all IndexerProperty calls