Update synth.py

[lauravoicu]

Summary

Fix CI fuzz

• Discrete outputs are integers: poisson, binomial, negbin now explicitly return np.int64.
• Negative binomial is robust: Gamma–Poisson mixture handles any positive k (dispersion), not just integer counts.
• Safer parameter checks: prevents NaNs/inf and catches degenerate quantiles for lognormal; validates shapes and scales.
• Shapes are consistent: always uses int(size) and returns 1-D arrays of length size (except Dirichlet, which stays (size, K)).

Type of change

• Fix
• Feature
• CI/CD
• Docs
• Chore