Improve test coverage with comprehensive edge case tests

- Add WGS84ConverterExtendedTests (12 tests):
  - Large offset coordinate conversions
  - Altitude changes
  - Extreme coordinates (poles, date line)
  - LatLonAltEstimate2 method
  - LatLonAltkm2UnityPos conversions
  - Edge cases for coordinate system conversions

- Add UWB2GPSConverterExtendedTests (10 tests):
  - Collinear beacons handling
  - Very close beacons
  - Invalid distance values (negative, zero)
  - Refinement algorithm edge cases (max iterations, zero learning rate)
  - Invalid latLonAlt data handling
  - Multiple unknown nodes
  - Isolated nodes with no neighbors

- Fix UWBManager null message handling:
  - Add explicit null check before JSON deserialization
  - Prevents ArgumentNullException from propagating

Coverage improvement:
- Line coverage: 58% → 59.4%
- Total tests: 92 → 112 (+20 tests)
- All tests passing ✅

Author: Alex J Lennon

.gitignore

@@ -85,3 +85,4 @@ build/
 .DS_Store
 Thumbs.db
 
+coverage/

src/UWBManager.cs

@@ -51,6 +51,11 @@ public static void UpdateUwbsFromMessage(string message)
     {
         try
         {
+            if (message == null)
+            {
+                _logger?.LogError("Message is null, cannot deserialize");
+                return;
+            }
             if (jsonOptions == null)
             {
                 _logger?.LogError("jsonOptions is null, cannot deserialize message");