[release/11.0] Fix Blazor client-side validation support for custom attributes

[github-actions]

Backport of #69020 to release/11.0

/cc @oroztocil

Fixes two problems in support for custom validation attributes in the Blazor SSR client-side validation feature.

Description

In .NET 11, we are adding a client-side validation feature for statically rendered Blazor forms. This feature allows static Blazor forms to give immediate validation feedback as the user is typing (i.e. without a full form submit). This is backed up by validation rules which are rendered into page based on .NET validation attributes and later enforced by JS code on the client.

The initial Preview release of the feature contains two problems in how user-defined validation attributes are supported:

1. The JS service that backs up the API for adding custom validators is initialized in a way that makes it hard or impossible to use the API in places where users would typically want to.
2. The .NET logic for emitting validation rule metadata into the page does not work properly with user-defined validation attributes in case where the custom attribute is derived from a built-in one (e.g. [Required]). This is a common scenario which needs to be supported.

Customer Impact

Without this fix, a new user-facing validation feature is incomplete or unnecessarily difficult to use with user-defined validation attributes. This would decrease the usefulness of the feature, and could be seen as a regression compared to a similar feature previously available in MVC, blocking MVC users from migrating to Blazor SSR.

Regression?

• Yes
• No

Risk

• High
• Medium
• Low

Two small fixes for a completely new feature.

Verification

• Manual (required)
• Automated

Packaging changes reviewed?

• Yes
• No
• N/A

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