test: add unit and integration tests for dual-track processing

Add comprehensive test suite for DirectExtractionEngine and dual-track
integration. All 65 tests pass covering text extraction, structure
preservation, routing logic, and backward compatibility.

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

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

openspec/changes/dual-track-document-processing/tasks.md

@@ -115,18 +115,18 @@
     - Note: UI prepared with disabled state; awaiting Section 5 implementation
 
 ## 8. Testing
-- [ ] 8.1 Unit tests for DocumentTypeDetector
-  - [ ] 8.1.1 Test various file types
-  - [ ] 8.1.2 Test editability detection
-  - [ ] 8.1.3 Test edge cases
-- [ ] 8.2 Unit tests for DirectExtractionEngine
-  - [ ] 8.2.1 Test text extraction accuracy
-  - [ ] 8.2.2 Test structure preservation
-  - [ ] 8.2.3 Test coordinate extraction
-- [ ] 8.3 Integration tests for dual-track processing
-  - [ ] 8.3.1 Test routing logic
-  - [ ] 8.3.2 Test UnifiedDocument generation
-  - [ ] 8.3.3 Test backward compatibility
+- [x] 8.1 Unit tests for DocumentTypeDetector
+  - [x] 8.1.1 Test various file types
+  - [x] 8.1.2 Test editability detection
+  - [x] 8.1.3 Test edge cases
+- [x] 8.2 Unit tests for DirectExtractionEngine
+  - [x] 8.2.1 Test text extraction accuracy
+  - [x] 8.2.2 Test structure preservation
+  - [x] 8.2.3 Test coordinate extraction
+- [x] 8.3 Integration tests for dual-track processing
+  - [x] 8.3.1 Test routing logic
+  - [x] 8.3.2 Test UnifiedDocument generation
+  - [x] 8.3.3 Test backward compatibility
 - [ ] 8.4 End-to-end tests
   - [ ] 8.4.1 Test scanned PDF processing (OCR track)
   - [ ] 8.4.2 Test editable PDF processing (direct track)