chore: remove dead code and add util tests

[adrioui]

Description of changes

• Remove dead code in visualize.py for output_type.__name__ fallback (was marked as TODO for removal)
• Add comprehensive tests for promote_list and promote_tuple utility functions (resolves TODO in test file)

[NickCrews]

remove this entire test:

• the dict case isn't being used
• we don't want to guarantee identity stability only equality stability

[NickCrews]

well, I guess we can add to the contract that identity is preserved.

but still, we don't need the paramaterization. just simplify to

def test_promote_list_identity():
    val = [1,2,3]
    assert promote_list(val) is val

[NickCrews]

Simplify to

def test_promote_tuple_identity():
    val = (1,2,3)
    assert promote_tuple(val) is val

[NickCrews]

Can you explain the impact of this change?

[adrioui]

Thanks for the review feedback! I've addressed all comments:

Test Simplifications (commits c94cbc7)

Simplified both identity tests per your suggestions:

test_promote_list_identity - Removed parameterization and dead code (the dict case was never actually tested due to the if isinstance(val, list) guard):

def test_promote_list_identity():
    """Test that lists are returned as-is (same object)."""
    val = [1, 2, 3]
    assert promote_list(val) is val

test_promote_tuple_identity - Removed unnecessary single-item parameterization:

def test_promote_tuple_identity():
    """Test that tuples are returned as-is (same object)."""
    val = (1, 2, 3)
    assert promote_tuple(val) is val

Impact Explanation for visualize.py Dead Code Removal

The output_type property was removed from all ops.Node classes in July 2022 (commit d2a3919). Since then, accessing node.output_type always raises AttributeError, making this code path unreachable.

The removed code and the remaining code produce identical behavior - returning "∅" for nodes without dtype or schema. Currently, the only node reaching this fallback is ops.JoinLink.

The existing test test_custom_expr_with_not_implemented_type in ibis/expr/tests/test_visualize.py verifies this fallback behavior continues to work correctly.