refactor: use using declarations for resource cleanup in LilToon tests

Replace try/finally cleanup patterns with C# 8 using var declarations
and DisposableObject<T> wrappers. Fixes CS0103 variable scope errors where
variables were declared inside try blocks but referenced in finally blocks.

- Fix CS0103 scope errors (5 in LitGeneratorTests, 6 in StandardGeneratorTests)
- Refactor all 5 test methods in LilToonToonLitGeneratorTests to using var
- Refactor all 13 test methods in LilToonToonStandardGeneratorTests to using var
- Replace UNT0008-triggering ?. null propagation on Unity objects with
  explicit != null ? ... : null conditional expressions
- Fix SA1118 multi-line parameter warning in Assert.Less call

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: kurotu