fix(prettify): render fractions and separators correctly

[rasheedja]

Summary

• Render fractional coefficients with their denominator. Previously, 1/2 was printed as 1 / 1.
• Use Data.List.intercalate to place + between terms, removing trailing separators from expressions, constraints, and objectives.
• Add positive and negative fraction coverage and update the existing formatting expectations. The empty-expression representation remains unchanged.

This PR targets master and has no dependency on the solver fixes.

Validation

• Updated formatting regressions failed before the fix.
• stack test --no-terminal: 272 examples, 0 failures.
• cabal check: no errors or warnings.
• Fourmolu 0.15.0.0 check of all tracked Haskell files passed.
• stack haddock --no-haddock-deps and git diff --check passed.