test: Add PDF/A validation tests and manual testing guide

- Add unit tests for ValidatePdfAQueryHandler (4 tests)
- Add integration tests for PDF/A validation endpoint (6 tests)
- Fix FluentValidation: Add Base64 format validation to ValidatePdfAQueryValidator
- Update AGENTS.md: Document 3-folder test structure rationale and test count (30 tests)
- Add DocumentOperator.API/README.md: 16 manual test scenarios for all endpoints

Test coverage:
- Unit: ValidatePdfAQueryHandler (compliant, non-compliant, encrypted, exceptions)
- Integration: PDF/A endpoint (multipart + Base64, validation, error handling)
- Manual: Step-by-step Swagger UI testing guide for all features

All 30 automated tests passing.

DocumentOperator.Application/ValidatePdfA/Validators/ValidatePdfAQueryValidator.cs

@@ -14,5 +14,35 @@ public class ValidatePdfAQueryValidator : AbstractValidator<Queries.ValidatePdfA
             .Must(x => (x.PdfBytes != null && x.PdfBytes.Length > 0) ^ 
                        !string.IsNullOrWhiteSpace(x.Base64Pdf))
             .WithMessage("Either PdfBytes or Base64Pdf must be provided, but not both");
+
+        When(x => !string.IsNullOrWhiteSpace(x.Base64Pdf), () =>
+        {
+            RuleFor(x => x.Base64Pdf!)
+                .Must(BeValidBase64)
+                .WithMessage("Base64Pdf must be a valid Base64 string");
+        });
+
+        When(x => x.PdfBytes != null, () =>
+        {
+            RuleFor(x => x.PdfBytes!)
+                .Must(bytes => bytes.Length > 0)
+                .WithMessage("PdfBytes cannot be empty");
+        });
+    }
+
+    private static bool BeValidBase64(string base64)
+    {
+        if (string.IsNullOrWhiteSpace(base64))
+            return false;
+
+        try
+        {
+            Convert.FromBase64String(base64);
+            return true;
+        }
+        catch (FormatException)
+        {
+            return false;
+        }
     }
 }