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

src/PyTexture.h

@@ -19,6 +19,7 @@ public:
     int sprite_width, sprite_height; // just use them read only, OK?
     PyTexture(std::string filename, int sprite_w, int sprite_h);
     sf::Sprite sprite(int index, sf::Vector2f pos = sf::Vector2f(0, 0), sf::Vector2f s = sf::Vector2f(1.0, 1.0));
+    int getSpriteCount() const { return sheet_width * sheet_height; }
 
     PyObject* pyObject();
     static PyObject* repr(PyObject*);

src/UISprite.cpp

@@ -151,6 +151,20 @@ int UISprite::set_int_member(PyUISpriteObject* self, PyObject* value, void* clos
         PyErr_SetString(PyExc_TypeError, "Value must be an integer.");
         return -1;
     }
+    
+    // Validate sprite index is within texture bounds
+    auto texture = self->data->getTexture();
+    if (texture) {
+        int sprite_count = texture->getSpriteCount();
+        
+        if (val < 0 || val >= sprite_count) {
+            PyErr_Format(PyExc_ValueError, 
+                "Sprite index %d out of range. Texture has %d sprites (0-%d)", 
+                val, sprite_count, sprite_count - 1);
+            return -1;
+        }
+    }
+    
     self->data->setSpriteIndex(val);
     return 0;
 }