Add unit tests for SetLiteral node

[BonnieZ-data]

Context

SetLiteral is a core node in the Wordplay language with no unit test coverage. This PR adds tests to verify that set literals evaluate correctly.

Tests Added

• Set literal with one element
• Set literal with multiple elements
• Set literal with string elements
• Set literal removes duplicate elements
• Set size via .size()
• Set is not equal to empty set
• Set is not empty
• Set union combines two sets
• Set intersection finds common elements
• Set difference removes elements

Why

SetLiteral had zero unit tests despite being a fundamental data structure in Wordplay. These tests ensure correct behavior and protect against regressions.

Related issues

• Closes Add unit tests for SetLiteral node #1123

[amyjko]

Thanks for the unit tests!