feat(core): add ContentServiceBase abstract class for Phase 1

Establishes shared infrastructure for content services:
- Common dependencies (DocumentRepository, AuditService, UserIdKeyResolver)
- Audit helper methods (sync and async)
- Inherits from RepositoryService for scope/query support
- Adds ContentServiceConstants for shared constants (batch page size)

Updated tracking test to look for correct assembly location.
Tracking test now fails (expected) - signals class exists for future work.

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

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

tests/Umbraco.Tests.UnitTests/Umbraco.Infrastructure/Services/ContentServiceBaseTests.cs

@@ -223,7 +223,7 @@ public class ContentServiceBaseTests
         // 2. Delete this tracking test
         // 3. Verify all tests pass
 
-        var type = Type.GetType("Umbraco.Cms.Infrastructure.Services.ContentServiceBase, Umbraco.Infrastructure");
+        var type = Type.GetType("Umbraco.Cms.Core.Services.ContentServiceBase, Umbraco.Core");
 
         Assert.That(type, Is.Null,
             "ContentServiceBase now exists! Uncomment all tests in this file and delete this tracking test.");