BUG: Fix Index.intersection returning same instance when indexes are equal GH#63169

[sudam802]

Fixes GH#63169.

Index.intersection previously returned the original Index instance
when both Index objects were equal and unique:

idx1 = Index([0, 1])
idx2 = Index([0, 1])
result = idx1.intersection(idx2)
result is idx1   # True (incorrect)

This caused identity issues when downstream code expected a new Index.

This PR ensures that a new Index object is always returned when
intersection is computed, even if both indexes are equal.

A new test has been added:

• test_intersection_returns_new_instance

This confirms:

• the result contains the correct values
• the returned Index is not the same object reference.