[ENH] Fix incorrect PMF plotting

[arnavk23]

Reference Issues/PRs

Fixes #416. See also #415.

What does this implement/fix? Explain your changes.

Fixes incorrect PMF plotting for discrete distributions. Previously, discrete distributions like Binomial were plotted as continuous curves. Now they correctly display as discrete probability mass functions using stem plots at integer support points.

Changes:

• Modified _plot_single() in BaseDistribution to detect discrete distributions
• Extract support ranges from scipy distributions and evaluate PMF at integer points
• Use stem plots for discrete PMF, maintain line plots for continuous distributions
• Handle infinite support bounds (e.g., Poisson)

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

No.

What should a reviewer concentrate their feedback on?

• Logic for discrete vs continuous distribution detection and plot type selection
• Support extraction and integer point evaluation
• Backward compatibility

Did you add any tests for the change?

Yes, added test_discrete_pmf_plotting() in test_proba_basic.py to verify stem plots are used for discrete PMF.

Any other comments?

Maintains full backward compatibility with existing continuous distribution plotting.

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