Support user callback with nutpie sampler

[saheli56]

Description

Resolves #8292.

Currently, passing callback to pm.sample with nuts_sampler=nutpie raises a ValueError. This PR lifts that restriction for nutpie by multiplexing the user callback alongside pb_manager.update via progress_callback.

Key points:

• When nuts_sampler=nutpie, the user callback is invoked with list[nutpie.ChainProgress] tracking per-chain sampling progress.
• Works consistently whether progressbar=True or progressbar=False.
• Non-nutpie external samplers (numpyro, blackjax) continue raising ValueError.
• Documented the callback parameter signature in pm.sample docstring.
• Added unit and regression tests in tests/sampling/test_mcmc_external.py.

Related Issue

Closes #8292

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Documentation build overview

📚 pymc | 🛠️ Build #35122604 | 📁 Comparing 76c8628 against latest (91e6da3)

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4 files changed

± glossary.html
± api/generated/pymc.sample.html
± _modules/pymc/sampling/mcmc.html
± _modules/pymc/dims/distributions/scalar.html