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.

AGENTS.md

@@ -188,7 +188,7 @@ After completing all Phase 1-3 controllers (PdfValidation, PdfAttachment, SwissQ
 dotnet build
 ```
 
-**Run tests (19 tests as of Feature 2):**
+**Run tests (30 tests as of Feature 3 - PDF/A Validation):**
 ```powershell
 dotnet test
 ```
@@ -273,7 +273,7 @@ Do NOT add `if (result.IsSuccess)` checks. Throw exceptions for errors. The midd
 
 | Controller | Status | Tests |
 |-----------|--------|-------|
-| **PdfValidationController** | ✅ Partial (validate done, validate-pdfa pending) | 4 |
+| **PdfValidationController** | ✅ DONE | 13 (7 validate + 6 validate-pdfa) |
 | **SwissQrCodeController** | ✅ DONE | 2 |
 | **PdfAttachmentController** | ⏳ Pending | 0 |
 | **PdfOperationsController** | ⏳ Pending | 0 |
@@ -333,6 +333,61 @@ var stream = Assembly.GetExecutingAssembly()
 
 ---
 
+## Test Structure & Strategy
+
+**3-folder structure (CORRECT approach by previous developer):**
+
+```
+DocumentOperator.Tests/
+├── Integration/
+│   └── API/
+│       ├── PdfValidationControllerTests.cs (13 tests)
+│       └── ExtractSwissQrCodeEndpointTests.cs (2 tests)
+├── TestData/
+│   └── Pdfs/ (EmbeddedResource PDFs)
+├── Unit/
+│   ├── Application/
+│   │   └── Features/
+│   │       ├── ValidatePdf/
+│   │       │   └── ValidatePdfHandlerTests.cs (2 tests)
+│   │       ├── ValidatePdfA/
+│   │       │   └── ValidatePdfAQueryHandlerTests.cs (4 tests)
+│   │       └── ExtractSwissQrCode/
+│   │           └── ExtractSwissQrCodeHandlerTests.cs (2 tests)
+│   └── Infrastructure/
+│       └── Services/
+│           └── PdfProcessing/
+│               └── DevExpressPdfProcessorTests.cs (7 tests)
+```
+
+**✅ Why this structure is CORRECT:**
+
+1. **Integration vs Unit separation:**
+   - **Integration:** WebApplicationFactory → REAL API calls (HTTP, middleware, MediatR pipeline, DevExpress)
+   - **Unit:** Mock-based ISOLATED tests (Handler only depends on mocked IPdfProcessor)
+
+2. **TestData centralization:**
+   - All 3 layers share same EmbeddedResource PDFs (no duplication)
+   - Accessed via `Assembly.GetManifestResourceStream()`
+
+3. **Vertical Slice compliance:**
+   - `Unit/Application/Features/ValidatePdf/` → Each feature's tests co-located
+   - Matches Application layer structure exactly
+
+4. **Test Pyramid:**
+   - **Unit tests (15):** Fast, isolated, many scenarios
+   - **Integration tests (15):** Slower, full pipeline, critical paths only
+
+**Test count:** 30 tests total (as of Feature 3 - PDF/A Validation)
+
+**FluentValidation in tests:**
+- Base64 format validation happens in `ValidatePdfQueryValidator` and `ValidatePdfAQueryValidator`
+- Prevents `FormatException` from reaching handler (caught as 400 Bad Request, not 500)
+- Unit tests verify handler behavior with valid inputs only
+- Integration tests verify full validation pipeline (including FluentValidation)
+
+---
+
 ## Swiss QR Code Feature (Feature 2)
 
 **Swiss QR Bill Standard 2.0** requires: