Address PR #485 review feedback: Add unique index on ExternalId, comprehensive tests, and fix test name

src/LinkDotNet.Blog.Infrastructure/Migrations/20260125214130_AddUniqueIndexOnExternalId.cs

@@ -0,0 +1,33 @@
+using System;
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace LinkDotNet.Blog.Infrastructure.Migrations;
+
+/// <inheritdoc />
+public partial class AddUniqueIndexOnExternalId : Migration
+{
+    /// <inheritdoc />
+    protected override void Up(MigrationBuilder migrationBuilder)
+    {
+        ArgumentNullException.ThrowIfNull(migrationBuilder);
+
+        migrationBuilder.CreateIndex(
+            name: "IX_BlogPosts_ExternalId",
+            table: "BlogPosts",
+            column: "ExternalId",
+            unique: true,
+            filter: "ExternalId IS NOT NULL");
+    }
+
+    /// <inheritdoc />
+    protected override void Down(MigrationBuilder migrationBuilder)
+    {
+        ArgumentNullException.ThrowIfNull(migrationBuilder);
+
+        migrationBuilder.DropIndex(
+            name: "IX_BlogPosts_ExternalId",
+            table: "BlogPosts");
+    }
+}

src/LinkDotNet.Blog.Infrastructure/Migrations/BlogDbContextModelSnapshot.cs

@@ -76,6 +76,11 @@ protected override void BuildModel(ModelBuilder modelBuilder)
 
                     b.HasKey("Id");
 
+                    b.HasIndex("ExternalId")
+                        .IsUnique()
+                        .HasDatabaseName("IX_BlogPosts_ExternalId")
+                        .HasFilter("ExternalId IS NOT NULL");
+
                     b.HasIndex("IsPublished", "UpdatedDate")
                         .IsDescending(false, true)
                         .HasDatabaseName("IX_BlogPosts_IsPublished_UpdatedDate");

src/LinkDotNet.Blog.Infrastructure/Persistence/Sql/Mapping/BlogPostConfiguration.cs

@@ -24,6 +24,11 @@ public void Configure(EntityTypeBuilder<BlogPost> builder)
         builder.Property(x => x.AuthorName).HasMaxLength(256).IsRequired(false);
         builder.Property(x => x.ExternalId).HasMaxLength(256).IsRequired(false);
 
+        builder.HasIndex(x => x.ExternalId)
+            .HasDatabaseName("IX_BlogPosts_ExternalId")
+            .IsUnique()
+            .HasFilter("ExternalId IS NOT NULL");
+
         builder.HasIndex(x => new { x.IsPublished, x.UpdatedDate })
             .HasDatabaseName("IX_BlogPosts_IsPublished_UpdatedDate")
             .IsDescending(false, true);