Binding select multiple

[guardrex]

Fixes #22448
Addresses #22448

🛑 HOLD FOR RELEASE OF PREVIEW 7 🛑 We're going to go ahead with the coverage BUT with a comment that it applies to Preview 7. This is blocking other work. I'll revert the added comment when Preview 7 releases (tracked by the 6.0 tracking issue).

• Internal Review Topic (Data Binding, links to section)
• Internal Review Topic (Forms and Validation, links to section)

On the PR:

• The multiple select feature for Preview 7 release. I'll hold this PR until Preview 7 comes out.
• Moves the null binding behavior section from the Forms Validation topic to the Data Binding topic and cross-links to it from Forms Validation. Cross-links also from each topic's new multiple select feature coverage.

Question ❓:

For InputSelect, the multiple attribute is inferred when bound to an array type. However, the example for binding <select> doesn't exhibit the same setup in the PU repo sample ...

https://github.com/dotnet/aspnetcore/blob/main/src/Components/test/testassets/BasicTestApp/SelectVariantsComponent.razor

Is inferred multiple a thing for binding <select>, too, and the sample just doesn't show it? If so, I'll mirror the InputSelect coverage remark in the <select> coverage and probably modify the <select> coverage example to drop multiple in that scenario, too (i.e., let it infer the attribute as the InputSelect example does).

[MackinnonBuck]

Looks great! Just one small suggestion 😃

[MackinnonBuck]

Also, no, <select> does not infer multiple.

Is inferred multiple a thing for binding , too, and the sample just doesn't show it?

[guardrex]

Thanks @MackinnonBuck! 🎷

I'll hold this PR for Preview 7 release.

[guardrex]

@MackinnonBuck ... This PR is blocking me from making other top priority updates for version-by-file (these topics need to move for that set of updates), so this PR has to go in now. I've added a line to each of the new sections that reads ...

This feature applies to ASP.NET Core 6.0 Preview 7 or later. ASP.NET Core 6.0 is scheduled for release later this year.

... and I've made a note on the 6.0 tracking issue to remove that line when Preview 7 releases.