Add data-file chain support to SamplingResult

[Fstarnb]

Summary

• Add serialization and loading for retained SamplingResult chains in JSON, MATLAB, and pickle files.
• Preserve the exact retained-chain shape, including zero retained iterations.
• Reject malformed shape metadata and report clearly when legacy files lack chain data.

Fixes #898.

Validation

• pytest tests/unit/test_sampling.py --unit -q — 88 passed
• ruff check pybop/_result.py pybop/samplers/base_sampler.py tests/unit/test_sampling.py
• ruff format --check pybop/_result.py pybop/samplers/base_sampler.py tests/unit/test_sampling.py
• python -m py_compile pybop/samplers/base_sampler.py tests/unit/test_sampling.py
• git diff --check

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.94%. Comparing base (3171a07) to head (5889a25).
⚠️ Report is 19 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #987      +/-   ##
===========================================
+ Coverage    92.13%   92.94%   +0.80%     
===========================================
  Files           74       84      +10     
  Lines         5799     6818    +1019     
===========================================
+ Hits          5343     6337     +994     
- Misses         456      481      +25     

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