Let calling code populate `ValidationContext.Items` when using Blazor or Minimal APIs

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Is your feature request related to a problem? Please describe the problem.

When writing custom data-annotations or using the `[CustomValidation]` attribute, it is sometimes useful to provide additional context to the validators. For example, to vary validation rules using "rule sets" (similar to FluentValidation).

The base data-annotations already provide this functionality. This feature request is about exposing that functionality in Blazor/Minimal APIs/MVC. The base `Validator.TryValidateObject` API lets me initialize the `ValidationContext` with the `Items` property.

**A concrete use case**
I have to a POCO declaring the data contract of an API endpoint. Depending on a query-string parameter, the client may save the POCO as a draft instead of submitting it. When validating a draft, I would like to only validate things such as maximum string lengths, but defer requiredness or minimum length validations until the submission. Using the `ValidationContext`'s `Items`property, I can add a custom key-value pair `{ "IsDraft": true }`.

When using Blazor's built-in validation or using Microsoft.Extensions.Validation to validate Minimal API endpoints, this is not possible.

### Describe the solution you'd like

When invoking Blazor's `EditContext.Validate()` method, I would like to pass a dictionary of context items to the `ValidationContext`.

When using Minimal APIs integration with Microsoft.Extensions.Validation, I would like to populate the context items before the endpoint filter runs.

The following ares examples of an API shape to start a conversation:

````csharp
// Blazor
var validationContextItems = new Dictionary()
{
["IsDraft"] = true
};

var isValid = EditContext.Validate(validationContextItems);
````

````csharp
// Minimal APIs
var builder = WebApplication.CreateBuilder(args);

var app = builder.Build();

app.MapGet(...).WithValidationContextItems((httpContext, validationContext) =>
{
if (HttpContext.Request.Query.TryGetValue("isDraft", out var stringValues) && bool.TryParse(stringValues[0], out var isDraft))
{
validationContext.Items.Add("IsDraft", isDraft);
}
else
{
validationContext.Items.Add("IsDraft", false);
}
})
````

# Alternatives

Don't use Blazor's built-in `` component or disable automatic validation using `.DisableValidation()` for Minimal APIs. In both cases, I need to inject the `ValidationOptions` to call `TryGetValidatableTypeInfo()` and initialize the `ValidationContext` with the items.

### Additional context

_No response_

Start by tracing Blazor's EditContext.Validate and DataAnnotationsValidator, then the Microsoft.Extensions.Validation integration for Minimal API endpoint filters. Compare how each initializes ValidationContext and determine a consistent way to supply Items. Done means the agreed API supports caller-provided context items in both validation paths, with behavior verified for custom validators.

Written by the indexing model from the issue text.