[BUG] reject non-positive sigma/scale in Normal and Laplace

[Ayushigitgithub]

Reference Issues/PRs

Closes #959

What does this implement/fix? Explain your changes.

Adds constructor-level validation for non-positive scale parameters in Normal and Laplace.

Changes:

• Normal now raises ValueError when sigma <= 0
• Laplace now raises ValueError when scale <= 0
• adds tests for:
  • Normal(mu=0, sigma=-1)
  • Normal(mu=0, sigma=0)
  • Laplace(mu=0, scale=-1)
  • Laplace(mu=0, scale=0)

Why is this needed?
Normal(mu=0, sigma=-1).pdf(0) previously returned -0.3989422804014327
Laplace(mu=0, scale=-1).pdf(0) previously returned -0.5

A pdf should never be negative, so these invalid parameters should be rejected early instead of silently producing mathematically invalid outputs.

Does your contribution introduce a new dependency? If yes, which one?

No

What should a reviewer concentrate their feedback on?

Constructor-level validation for non-positive sigma / scale in Normal and Laplace, and whether the added regression tests in test_proba_basic.py are the right place and scope for this fix.

Did you add any tests for the change?

Yes.

Added tests for:

Normal(mu=0, sigma=-1)
Normal(mu=0, sigma=0)
Laplace(mu=0, scale=-1)
Laplace(mu=0, scale=0)

These verify that non-positive scale parameters are rejected with ValueError.

Any other comments?

Closes #959

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