Add jitter_scale parameter for initial point generation

[aphc14]

Description

Add jitter_scale parameter to control the bounds of uniform jitter in initial points. This allows users to adjust the width of random initialisation, which is useful for the Multi-path Pathfinder algorithm to explore posteriors in a larger parameter space or where the posterior mean is much further away from the prior mean.

Changes:

• Add jitter_scale parameter (default=1.0) to initial point functions
• Update jitter bounds from U(-1,1) to U(-jitter_scale, jitter_scale)
• Add test_jitter_scale to verify scaling behaviour
• Update docstrings to reflect new parameter

Related Issue

• [x] Closes #7555
• [x] Related to Pathfinder PR in pymc-extras

Checklist

• [x] Checked that the pre-commit linting/style checks pass
• [x] Included tests that prove the fix is effective or that the new feature works
• [ ] 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
• [ ] Other (please specify):

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📚 Documentation preview 📚: https://pymc--7643.org.readthedocs.build/en/7643/

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