Add [NotNullIfNotNull] to EnumerableExtensions.Random<T> for better nullable flow analysis

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: niemyjski

AGENTS.md

@@ -78,6 +78,7 @@ test
 - **`Math.Clamp`**: Use instead of separate `Math.Min`/`Math.Max` calls
 - **Generic constraints**: Use `where T : struct, Enum` instead of runtime `typeof(T).IsEnum` checks
 - **Pattern matching**: Use `is null` / `is not null` instead of `== null` / `!= null`
+- **Nullable flow attributes**: Use `[NotNullIfNotNull]`, `[NotNullWhen]`, `[MaybeNullWhen]`, `[AllowNull]` from `System.Diagnostics.CodeAnalysis` to express conditional nullability contracts that the compiler can track through call sites — prefer these over `T?` return types when nullability depends on a specific argument
 
 ### Exceptions
 

src/Exceptionless.RandomData/RandomData.cs

@@ -1,3 +1,4 @@
+using System.Diagnostics.CodeAnalysis;
 using System.Security.Cryptography;
 using System.Text;
 
@@ -380,7 +381,9 @@ public static string GetParagraphs(int count = 3, int minSentences = 3, int maxS
 public static class EnumerableExtensions {
     /// <summary>
     /// Returns a random element from <paramref name="items"/>, or <paramref name="defaultValue"/> if the sequence is null or empty.
+    /// The return value is non-null when <paramref name="defaultValue"/> is non-null.
     /// </summary>
+    [return: NotNullIfNotNull(nameof(defaultValue))]
     public static T? Random<T>(this IEnumerable<T>? items, T? defaultValue = default) {
         if (items is null)
             return defaultValue;