fix(validation): make Severity meaningful in Validation.IsValid

Validation.IsValid previously counted every problem regardless of severity,
leaving Severity inert. Now IsValid only fails on Error problems, so Warning
and Info no longer invalidate the result, and IsValidFor(Severity) lets
callers set the failing-severity threshold.

src/request.validation/Validation.cs

@@ -23,7 +23,27 @@ public sealed class Validation
 	/// <summary>
 	/// Whether the validation was successful.
 	/// </summary>
-	public bool IsValid => Problems.Count is 0;
+	/// <remarks>
+	/// A validation is considered successful unless it contains at least one problem with
+	/// <see cref="Severity.Error"/>. Problems of lower severity (<see cref="Severity.Warning"/> or
+	/// <see cref="Severity.Info"/>) do not invalidate the result. Use <see cref="IsValidFor"/>
+	/// to control which severities are treated as failures.
+	/// </remarks>
+	public bool IsValid => IsValidFor(Severity.Error);
+
+	/// <summary>
+	/// Whether the validation was successful, treating problems at or above the given severity as failures.
+	/// </summary>
+	/// <param name="minimum">The minimum severity that should be considered a failure.</param>
+	/// <remarks>
+	/// A problem invalidates the validation when its <see cref="Severity"/> is at least as severe as
+	/// <paramref name="minimum"/>. For example, <c>IsValidFor(Severity.Warning)</c> fails on errors and
+	/// warnings but not on informational problems.
+	/// </remarks>
+	public bool IsValidFor(Severity minimum)
+	{
+		return !Problems.Any(problem => problem.Severity <= minimum);
+	}
 
 	/// <summary>
 	/// The problems that were found during validation.