Refactor: Rename DocumentOperator to DocumentService

Updated project and solution files to rename and restructure
`DocumentOperator` to `DocumentService`. Modified project
references in `DocumentService.API.csproj`, `DocumentService.Application.csproj`,
and `DocumentService.Infrastructure.csproj` to reflect the new
naming convention. Updated `DocumentService.sln` to remove
old project references, add new ones, and adjust configuration
and nested project mappings accordingly. This change aligns
the project structure with the new naming convention.

DocumentService.Domain/Models/ValueObjects/TenantId.cs

@@ -0,0 +1,41 @@
+namespace DocumentService.Domain.Models.ValueObjects;
+
+public sealed class TenantId
+{
+    public string Value { get; }
+
+    private TenantId(string value)
+    {
+        Value = value;
+    }
+
+    public static TenantId Create(string value)
+    {
+        if (string.IsNullOrWhiteSpace(value))
+            throw new Common.Exceptions.DomainValidationException("TenantId cannot be empty.");
+
+        if (value.Length > 100)
+            throw new Common.Exceptions.DomainValidationException("TenantId cannot exceed 100 characters.");
+
+        // Normalisierung: Lowercase
+        var normalized = value.Trim().ToLowerInvariant();
+
+        return new TenantId(normalized);
+    }
+
+    public override string ToString() => Value;
+
+    // Equality
+    public override bool Equals(object? obj)
+    {
+        if (obj is not TenantId other)
+            return false;
+
+        return Value == other.Value;
+    }
+
+    public override int GetHashCode()
+    {
+        return Value.GetHashCode();
+    }
+}