Show correct extra item in sequence comparison diffs - #14906

pytest reported the wrong extra item when comparing sequences of different lengths. It printed the item past the end of the shorter side. The real extra item sat at the first differing index.
Before:

E     assert [1, 2, 3] == [1, 2, 0, 3]
E       At index 2 diff: 3 != 0
E       Right contains one more item: 3   (wrong: extra item is 0)

After:

E     Right contains one more item: 0

The fix finds a single inserted item by checking whether the tails after the first differing index align, offset by one. It corrects only the single-extra-item case. Multi-item length differences still use the previous "first extra item" message.

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• Closes Misleading message "Right contains one more item" #13652
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