refactor(distributions): standardize output types

[xImoZA]

This PR standardizes the behavior of distributions to ensure consistent return types (scalars vs arrays) and improves the numerical stability of PDF calculations.

Changes

• Numerical Stability: Refactored pdf methods in all distributions to calculate probability density via np.exp(lpdf(X)).
• Standardized Outputs: Updated distribution methods (pdf, lpdf, ppf, log_gradients) to strictly distinguish between inputs:
  • Scalar input -> Scalar output.
  • Array input -> Array output with correct dtype.
• Testing: Added comprehensive property-based tests (via Hypothesis) for all distributions ensuring that:
  • Scalar inputs return scalars (np.isscalar check).
  • Array inputs return arrays with correct shapes and dtypes.