Add mutation for ternary operators

[ryanfreckleton]

Closes #196

This adds mutations for ternary expressions (a if cond else b), as discussed in the issue.

The new mutations target the IfExp condition and force each branch explicitly:

mark = "x" if count % 2 else "o"

can now mutate to:

mark = "x" if count % 2 and False else "o"
mark = "x" if count % 2 or True else "o"

Implementation notes:

• adds an IfExp mutation operator in node_mutation.py
• uses LibCST's IfExp node, following the implementation pointer in the issue comments
• wraps low-precedence conditions like b or c before appending and False / or True, so we preserve the original condition semantics

I kept this scoped to ternary operators only, not normal if statements, which matches the issue rationale that branch coverage is less helpful for inline conditionals.

Tests:

• added direct mutation tests for ternary expressions
• added a precedence-preservation test for boolean conditions inside ternaries
• updated the existing mutated-function assertion to check for the new ternary mutants without depending on exact mutant numbering

[Otto-AA]

Thank you for the PR!

It's an interesting question, but If I'm not mistaken, I don't think the precedence matters in these cases.

For both 1 if (b or c) and False else 2 and 1 if b or (c and False) else 2:

• we always evaluate expression b first
• if b is falsy, we always evaluate c
• we always return False

In the general case, I think adding and False or or True won't effect which expressions we evaluate, regardless of precedence.

Are you aware of a case where it could matter? Then it would be nice to add it here as test case. Otherwise, I think we could simplify the operator_if_exp mutation to directly use node.test.