Add background sampling handle to pm.sample

[XPE-7]

Description

This adds an opt-in background mode to pm.sample so users can start sampling and continue working while it runs in a background thread. When background=True, pm.sample returns a handle with done(), result(), and exception() helpers, and the progress bar is suppressed to keep output clean. Background mode is currently limited to the built-in nuts_sampler="pymc"; other samplers raise NotImplementedError to make the scope clear. The model is resolved eagerly so the background thread always has a valid context.

Docstring notes describe how to use background=True and the current limitations. Tests cover both a successful background run and error propagation (tests/sampling/test_background_sampling.py).

Summary of changes:

• Add background flag to pm.sample and return a BackgroundSampleHandle when enabled.
• Suppress progress bars in background mode.
• Resolve the model before launching the background thread.
• Document background mode in the pm.sample docstring.
• Add tests for background sampling (happy path and error propagation).

Related Issue

• [x] Closes #7929

Checklist

• [ ] Checked that pre-commit linting/style checks pass
• [x] Included tests that prove the fix is effective or that the new feature works
• [x] Added necessary documentation (docstrings and/or example notebooks)
• [ ] If you are a pro: each commit corresponds to a relevant logical change

Type of change

• [x] New feature / enhancement
• [ ] Bug fix
• [ ] Documentation
• [ ] Maintenance