Implement sprite index validation for Issue #33

Added validation to prevent setting sprite indices outside the valid
range for a texture. The implementation:
- Adds getSpriteCount() method to PyTexture to expose total sprites
- Validates sprite_number setter to ensure index is within bounds
- Provides clear error messages showing valid range
- Works for both Sprite and Entity objects

closes #33

ROADMAP.md

@@ -200,8 +200,8 @@ Created comprehensive test suite with 13 tests covering all Python-exposed metho
 8. ✅ Fix PyVector x/y properties - Were returning None
 
 REMAINING IN PHASE 1:
-9. Fix #73 - Entity.index() method for removal
-10. Fix #27 - EntityCollection.extend() method
+9. ✅ Fix #73 - Entity.index() method for removal
+10. ✅ Fix #27 - EntityCollection.extend() method
 11. Fix #33 - Sprite index validation
 12. Alpha Blockers (#3, #2) - Remove deprecated methods
 ```