use safe negative binomial rng

[sbfnk]

Description

This PR closes #963 by implementing a safe version of the negative binomial 2 rng. Existing code for avoiding numerical issues with this has been moved into the function.

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After the initial Pull Request

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[github-actions]

Thank you for your contribution sbfnk 🚀! Your synthetic_recovery markdown is ready for download 👉 here 👈!
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[github-actions]

This is how benchmark results would change (along with a 95% confidence interval in relative change) if 3104110 is merged into main:

• ✔️default: 22.9s -> 22.3s [-12.4%, +7.23%]
• ✔️no_delays: 23.6s -> 23.6s [-12.32%, +12.62%]
• ✔️random_walk: 9.46s -> 8.96s [-12.64%, +1.91%]
• ✔️stationary: 12s -> 12.1s [-13.52%, +14.58%]
• ✔️uncertain: 50.3s -> 36.2s [-90.08%, +34.19%]
  Further explanation regarding interpretation and methodology can be found in the documentation.