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

quick_sprite_test.py

@@ -0,0 +1,25 @@
+import mcrfpy
+
+# Test sprite index validation
+t = mcrfpy.Texture("assets/kenney_ice.png", 16, 16)
+s = mcrfpy.Sprite(10, 10, t, 5)
+
+print(f"Initial sprite index: {s.sprite_number}")
+
+# Try valid index
+s.sprite_number = 50
+print(f"Set to 50: {s.sprite_number}")
+
+# Try invalid index
+try:
+    s.sprite_number = 200
+    print("ERROR: Should have rejected index 200")
+except ValueError as e:
+    print(f"✓ Correctly rejected: {e}")
+
+# Try negative
+try:
+    s.sprite_number = -1
+    print("ERROR: Should have rejected negative index")
+except ValueError as e:
+    print(f"✓ Correctly rejected negative: {e}")