Add tests for category_map argument in detect() / _convert_detections()

[Copilot]

No tests existed to verify that category_map is correctly applied to produce category_label in detection results.

New test classes

TestCategoryMapConvertDetections — exercises _convert_detections directly:

• Custom map entry produces the correct category_label
• All entries in a custom map are applied correctly
• Category ID absent from the map yields "unknown"
• category_map=None falls back to MD_LABELS
• Explicit MD_LABELS produces correct labels for mdv5 (category offset included)

TestDetectCategoryMap — exercises detect() via mocking:

• detect() forwards the caller-supplied category_map as the 5th positional arg to _convert_detections
• Returned category_label values reflect the custom map

custom_map = {0: 'fox', 1: 'rabbit'}
results = detect(detector, 'image.jpg', 640, 640, category_map=custom_map)
# results[0]['detections'][0]['category_label'] == 'fox'