fix: fragile/vacuous assertions in oscat, codesys-import, located-variables tests (#75)

[michaeldistel]

Closes #75

Summary

• Replace silent return with expect.fail() in oscat second test so it hard-fails if transpilation output is missing
• Replace brittle exact-equality FUNCTION/TYPE count assertions in codesys-import V3 vs V2.3 comparison with toBeGreaterThan(0)
• Add LOCATED_VAR_IN_FB error code to semantic analyzer; thread code through addError and index.ts error mapping; assert on e.code in located-variables test instead of fragile string-contains

Testing

• npm test: 51 test files, 1414 passed, 7 skipped — all passing