feat(testing): implement Phase 4 advanced testing

- Add Hypothesis property-based tests (19 tests)
- Add Syrupy snapshot tests (7 tests)
- Add pytest-benchmark performance tests (11 tests)
- Fix Hypothesis plugin compatibility by creating MockModule class
- Update pytest.ini to exclude .hypothesis directory
- Add .hypothesis/ to .gitignore
- Update requirements-dev.txt with testing dependencies
- Mark Phase 4 complete in backend-testing-improvement-plan.md

All 947 tests passing.

docs/testing/backend-testing-improvement-plan.md

@@ -1,6 +1,6 @@
 # Backend Testing Improvement Plan
 
-**Status:** Phase 3 Complete ✅  
+**Status:** Phase 4 Complete ✅  
 **Created:** 2026-02-11  
 **Updated:** 2026-02-11  
 **Priority:** P0 - Critical
@@ -340,6 +340,54 @@ assert len(ws_manager.payloads) >= 2  # Started + completed
 
 ---
 
+## Phase 4 Completion Summary (2026-02-11)
+
+### Completed Items
+
+1. **Property-Based Tests (Hypothesis)** ✅
+   - Created `tests/utils/test_utils_hypothesis.py` with 19 property-based tests
+   - Tests cover:
+     - `sanitize_folder_name` idempotency and invalid character handling (4 tests)
+     - `_sanitize_library_name` idempotency and safe character filtering (2 tests)
+     - `normalize_path` idempotency and forward slash usage (2 tests)
+     - `fuzzy_match` edge cases and threshold behavior (3 tests)
+     - `determine_base_model` return type guarantees (2 tests)
+     - `get_preview_extension` return type validation (2 tests)
+     - `calculate_recipe_fingerprint` determinism and ordering (4 tests)
+   - Fixed Hypothesis plugin compatibility issue by creating a `MockModule` class in `conftest.py` that is hashable (unlike `types.SimpleNamespace`)
+
+2. **Snapshot Tests (Syrupy)** ✅
+   - Created `tests/routes/test_api_snapshots.py` with 7 snapshot tests
+   - Tests cover:
+     - SettingsHandler response formats (2 tests)
+     - NodeRegistryHandler response formats (2 tests)
+     - Utility function output verification (2 tests)
+     - ModelLibraryHandler empty response format (1 test)
+   - All snapshots generated and tests passing (7/7)
+
+3. **Performance Benchmarks** ✅
+   - Created `tests/performance/test_cache_performance.py` with 11 benchmark tests
+   - Tests cover:
+     - Hash index lookup performance (100, 1K, 10K models) - 3 tests
+     - Hash index add entry performance (100, 10K existing) - 2 tests
+     - Fuzzy matching performance (short text, long text, many words) - 3 tests
+     - Recipe fingerprint calculation (5, 50, 200 LoRAs) - 3 tests
+   - All benchmarks passing with performance metrics (11/11)
+
+4. **Package Dependencies** ✅
+   - Added `hypothesis>=6.0` to `requirements-dev.txt`
+   - Added `syrupy>=5.0` to `requirements-dev.txt`
+   - Added `pytest-benchmark>=5.0` to `requirements-dev.txt`
+
+### Test Results
+- **Property-Based Tests:** 19/19 passing
+- **Snapshot Tests:** 7/7 passing
+- **Performance Benchmarks:** 11/11 passing
+- **Total New Tests Added:** 37 tests
+- **Full Test Suite:** 947/947 passing
+
+---
+
 ## Phase 3 Completion Summary (2026-02-11)
 
 ### Completed Items
@@ -569,12 +617,12 @@ def test_cache_lookup_performance(benchmark):
 - [x] Remove duplicate singleton reset fixtures (consolidated in conftest.py)
 
 ### Week 7-8: Advanced Testing
-- [ ] Install hypothesis
-- [ ] Add 10 property-based tests
-- [ ] Install syrupy
-- [ ] Add 5 snapshot tests
-- [ ] Install pytest-benchmark
-- [ ] Add 3 performance benchmarks
+- [x] Install hypothesis (Added to requirements-dev.txt)
+- [x] Add 10 property-based tests (Created 19 tests in test_utils_hypothesis.py)
+- [x] Install syrupy (Added to requirements-dev.txt)
+- [x] Add 5 snapshot tests (Created 7 tests in test_api_snapshots.py)
+- [x] Install pytest-benchmark (Added to requirements-dev.txt)
+- [x] Add 3 performance benchmarks (Created 11 tests in test_cache_performance.py)
 
 ---