Add doctests to change_brightness

Added comprehensive doctests for change_brightness function.

Contributes to #9943

Author: adrian-cancio

digital_image_processing/change_brightness.py

@@ -4,6 +4,33 @@
 def change_brightness(img: Image, level: float) -> Image:
     """
     Change the brightness of a PIL Image to a given level.
+
+    Args:
+        img: PIL Image to adjust
+        level: Brightness level adjustment (-255.0 to 255.0)
+               Negative values darken, positive values brighten
+
+    Returns:
+        New PIL Image with adjusted brightness
+
+    Raises:
+        ValueError: If level is not in range [-255.0, 255.0]
+
+    >>> from PIL import Image
+    >>> import numpy as np
+    >>> img = Image.new('RGB', (2, 2), color=(100, 100, 100))
+    >>> bright = change_brightness(img, 50)
+    >>> px = bright.load()
+    >>> px[0, 0]
+    (150, 150, 150)
+    >>> dark = change_brightness(img, -50)
+    >>> px_dark = dark.load()
+    >>> px_dark[0, 0]
+    (50, 50, 50)
+    >>> change_brightness(img, 300)
+    Traceback (most recent call last):
+        ...
+    ValueError: level must be between -255.0 (black) and 255.0 (white)
     """
 
     def brightness(c: int) -> float: