Fix Cesium entity ID collision bug (issue #136)

[giswqs]

Problem

Fixes issue #136 where adding multiple landmarks/entities to a Cesium map results in the error:
An entity with id point_0 already exists in this collection.

Root Cause

The existing code generates entity IDs using len(self._layers) which causes ID collisions because:

1. Different entity types (points, billboards, polylines, polygons) can generate the same numeric suffix
2. If entities are removed, len(self._layers) can decrease and reuse previous IDs
3. The length-based approach doesn't guarantee unique IDs across the entire entity collection

Solution

• Added entity counter: Introduced self._entity_counter that only increments, never decreases
• Unique ID generator: Added _generate_entity_id(prefix) method that creates guaranteed unique IDs
• Fixed all entity types: Updated add_point, add_billboard, add_polyline, and add_polygon methods
• Prevents ID reuse: Counter-based approach ensures no ID collision even after entity removal

Changes

• anymap/cesium.py:
  • Initialize _entity_counter = 0 in constructor
  • Add _generate_entity_id(prefix) method
  • Replace f"{type}_{len(self._layers)}" with self._generate_entity_id(type) in all entity creation methods

Testing

Verified the fix works correctly:

• Generated multiple entities of different types
• Confirmed all IDs are unique: ['point_0', 'point_1', 'billboard_2', 'polyline_3', 'polygon_4']
• Tested edge cases like entity removal followed by new entity creation
• No ID collisions occur

Impact

• ✅ Resolves the 'entity already exists' error when adding multiple landmarks
• ✅ Maintains backward compatibility (same ID format, just guaranteed unique)
• ✅ No breaking changes to existing API
• ✅ Applies to all Cesium entity types consistently

Closes #136

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Pull request overview

This pull request fixes a critical bug in Cesium entity ID generation that caused "entity already exists" errors when adding multiple landmarks or entities to a Cesium map (issue #136).

Changes:

• Introduced a monotonically-incrementing _entity_counter to ensure unique entity IDs
• Added _generate_entity_id() helper method for consistent ID generation across all entity types
• Updated all entity creation methods (add_point, add_billboard, add_polyline, add_polygon) to use the new ID generator

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This fix for entity ID uniqueness should include a corresponding test case. Both LeafletMap and OpenLayersMap have test_layer_id_generation tests (see tests/test_leaflet.py:271 and tests/test_openlayers.py:313) that verify ID uniqueness by adding multiple entities and confirming all IDs are unique and properly prefixed. Consider adding a similar test_entity_id_generation test to tests/test_anymap.py in the TestCesiumMap class that creates multiple entities of different types and verifies all generated IDs are unique and don't collide.