feat(core): add IContentQueryOperationService interface for Phase 2

Extracts query operations (Count, GetByLevel, GetPagedOfType/s) into
focused interface following Phase 1 patterns.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/Umbraco.Core/Services/IContentQueryOperationService.cs

@@ -0,0 +1,123 @@
+// src/Umbraco.Core/Services/IContentQueryOperationService.cs
+using Umbraco.Cms.Core.Models;
+using Umbraco.Cms.Core.Persistence.Querying;
+
+namespace Umbraco.Cms.Core.Services;
+
+/// <summary>
+/// Service for content query operations (counting, filtering by type/level).
+/// </summary>
+/// <remarks>
+/// <para>
+/// <strong>Implementation Note:</strong> Do not implement this interface directly.
+/// Instead, inherit from <see cref="ContentServiceBase"/> which provides required
+/// infrastructure (scoping, repository access, auditing). Direct implementation
+/// without this base class will result in missing functionality.
+/// </para>
+/// <para>
+/// This interface is part of the ContentService refactoring initiative (Phase 2).
+/// It extracts query operations into a focused, testable service.
+/// </para>
+/// <para>
+/// <strong>Versioning Policy:</strong> This interface follows additive-only changes.
+/// New methods may be added with default implementations. Existing methods will not
+/// be removed or have signatures changed without a 2 major version deprecation period.
+/// </para>
+/// <para>
+/// <strong>Version History:</strong>
+/// <list type="bullet">
+///   <item><description>v1.0 (Phase 2): Initial interface with Count, GetByLevel, GetPagedOfType operations</description></item>
+/// </list>
+/// </para>
+/// </remarks>
+/// <since>1.0</since>
+public interface IContentQueryOperationService : IService
+{
+    #region Count Operations
+
+    /// <summary>
+    /// Counts content items, optionally filtered by content type.
+    /// </summary>
+    /// <param name="contentTypeAlias">Optional content type alias to filter by. If the alias doesn't exist, returns 0.</param>
+    /// <returns>The count of matching content items (includes trashed items).</returns>
+    int Count(string? contentTypeAlias = null);
+
+    /// <summary>
+    /// Counts published content items, optionally filtered by content type.
+    /// </summary>
+    /// <param name="contentTypeAlias">Optional content type alias to filter by. If the alias doesn't exist, returns 0.</param>
+    /// <returns>The count of matching published content items.</returns>
+    int CountPublished(string? contentTypeAlias = null);
+
+    /// <summary>
+    /// Counts children of a parent, optionally filtered by content type.
+    /// </summary>
+    /// <param name="parentId">The parent content id. If the parent doesn't exist, returns 0.</param>
+    /// <param name="contentTypeAlias">Optional content type alias to filter by. If the alias doesn't exist, returns 0.</param>
+    /// <returns>The count of matching child content items.</returns>
+    int CountChildren(int parentId, string? contentTypeAlias = null);
+
+    /// <summary>
+    /// Counts descendants of an ancestor, optionally filtered by content type.
+    /// </summary>
+    /// <param name="parentId">The ancestor content id. If the ancestor doesn't exist, returns 0.</param>
+    /// <param name="contentTypeAlias">Optional content type alias to filter by. If the alias doesn't exist, returns 0.</param>
+    /// <returns>The count of matching descendant content items.</returns>
+    int CountDescendants(int parentId, string? contentTypeAlias = null);
+
+    #endregion
+
+    #region Hierarchy Queries
+
+    /// <summary>
+    /// Gets content items at a specific tree level.
+    /// </summary>
+    /// <param name="level">The tree level (1 = root children, 2 = grandchildren, etc.).</param>
+    /// <returns>Content items at the specified level, excluding trashed items.</returns>
+    IEnumerable<IContent> GetByLevel(int level);
+
+    #endregion
+
+    #region Paged Type Queries
+
+    /// <summary>
+    /// Gets paged content items of a specific content type.
+    /// </summary>
+    /// <param name="contentTypeId">The content type id. If the content type doesn't exist, returns empty results with totalRecords = 0.</param>
+    /// <param name="pageIndex">Zero-based page index.</param>
+    /// <param name="pageSize">Page size.</param>
+    /// <param name="totalRecords">Output: total number of matching records.</param>
+    /// <param name="filter">Optional filter query.</param>
+    /// <param name="ordering">Optional ordering (defaults to sortOrder).</param>
+    /// <returns>Paged content items.</returns>
+    /// <exception cref="ArgumentOutOfRangeException">Thrown when pageIndex is negative or pageSize is less than or equal to zero.</exception>
+    IEnumerable<IContent> GetPagedOfType(
+        int contentTypeId,
+        long pageIndex,
+        int pageSize,
+        out long totalRecords,
+        IQuery<IContent>? filter = null,
+        Ordering? ordering = null);
+
+    /// <summary>
+    /// Gets paged content items of multiple content types.
+    /// </summary>
+    /// <param name="contentTypeIds">The content type ids. If empty or containing non-existent IDs, returns empty results with totalRecords = 0.</param>
+    /// <param name="pageIndex">Zero-based page index.</param>
+    /// <param name="pageSize">Page size.</param>
+    /// <param name="totalRecords">Output: total number of matching records.</param>
+    /// <param name="filter">Optional filter query.</param>
+    /// <param name="ordering">Optional ordering (defaults to sortOrder).</param>
+    /// <returns>Paged content items.</returns>
+    /// <exception cref="ArgumentNullException">Thrown when contentTypeIds is null.</exception>
+    /// <exception cref="ArgumentOutOfRangeException">Thrown when pageIndex is negative or pageSize is less than or equal to zero.</exception>
+    IEnumerable<IContent> GetPagedOfTypes(
+        int[] contentTypeIds,
+        long pageIndex,
+        int pageSize,
+        out long totalRecords,
+        IQuery<IContent>? filter = null,
+        Ordering? ordering = null);
+
+    #endregion
+}

tests/Umbraco.Tests.UnitTests/Umbraco.Core/Services/ContentQueryOperationServiceInterfaceTests.cs

@@ -0,0 +1,30 @@
+// tests/Umbraco.Tests.UnitTests/Umbraco.Core/Services/ContentQueryOperationServiceInterfaceTests.cs
+using NUnit.Framework;
+using Umbraco.Cms.Core.Services;
+
+namespace Umbraco.Cms.Tests.UnitTests.Umbraco.Core.Services;
+
+[TestFixture]
+public class ContentQueryOperationServiceInterfaceTests
+{
+    [Test]
+    public void IContentQueryOperationService_Interface_Exists()
+    {
+        // Arrange & Act
+        var interfaceType = typeof(IContentQueryOperationService);
+
+        // Assert
+        Assert.That(interfaceType, Is.Not.Null);
+        Assert.That(interfaceType.IsInterface, Is.True);
+    }
+
+    [Test]
+    public void IContentQueryOperationService_Extends_IService()
+    {
+        // Arrange
+        var interfaceType = typeof(IContentQueryOperationService);
+
+        // Act & Assert
+        Assert.That(typeof(IService).IsAssignableFrom(interfaceType), Is.True);
+    }
+}