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.Application/Common/DTOs/PdfAValidationResult.cs

@@ -0,0 +1,52 @@
+namespace DocumentService.Application.Common.DTOs;
+
+/// <summary>
+/// PDF/A validation result including conformance level and validation errors/warnings
+/// </summary>
+public record PdfAValidationResult
+{
+    /// <summary>
+    /// Whether the PDF is valid (no errors)
+    /// </summary>
+    public bool IsValid { get; init; }
+
+    /// <summary>
+    /// PDF version (e.g., "1.4", "1.7")
+    /// </summary>
+    public string PdfVersion { get; init; } = string.Empty;
+
+    /// <summary>
+    /// Number of pages in the PDF
+    /// </summary>
+    public int PageCount { get; init; }
+
+    /// <summary>
+    /// File size in bytes
+    /// </summary>
+    public long FileSize { get; init; }
+
+    /// <summary>
+    /// Whether the PDF is encrypted
+    /// </summary>
+    public bool Encrypted { get; init; }
+
+    /// <summary>
+    /// PDF/A version (e.g., "PDF/A-1b", "PDF/A-2a", "PDF/A-3u") or null if not PDF/A compliant
+    /// </summary>
+    public string? PdfAVersion { get; init; }
+
+    /// <summary>
+    /// Whether the PDF conforms to PDF/A standard
+    /// </summary>
+    public bool PdfACompliant { get; init; }
+
+    /// <summary>
+    /// Validation errors (e.g., "PDF/A documents cannot be encrypted")
+    /// </summary>
+    public IReadOnlyList<string> Errors { get; init; } = [];
+
+    /// <summary>
+    /// Validation warnings (e.g., "Manual verification recommended: All fonts must be embedded")
+    /// </summary>
+    public IReadOnlyList<string> Warnings { get; init; } = [];
+}