Fix edge-case validation in nuts_params.list()

[utkarshpawade]

Fixes #530

• Add early validation in nuts_params.list() for empty lists and zero-row matrices with clear error messages
• Add unit tests for empty list, zero-row matrix, and single-chain input edge cases

[Copilot]

Pull request overview

This PR addresses Issue #530 by adding explicit input validation to nuts_params.list() so edge-case inputs (empty lists and zero-row matrices) fail with clear, user-facing errors rather than downstream/internal failures.

Changes:

• Add early validation in nuts_params.list() for empty list inputs and zero-row matrices.
• Add unit tests covering empty list, zero-row matrix, and single-chain list behavior.
• Document the behavioral change in NEWS.md.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
R/bayesplot-extractors.R	Adds early input validation and clearer error messages in nuts_params.list().
tests/testthat/test-extractors.R	Adds regression tests for new edge-case validations and a single-chain success case.
NEWS.md	Notes the new validation behavior in the development changelog.

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.67%. Comparing base (62fce06) to head (6238977).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #531      +/-   ##
==========================================
- Coverage   98.71%   98.67%   -0.04%     
==========================================
  Files          35       35              
  Lines        5903     5907       +4     
==========================================
+ Hits         5827     5829       +2     
- Misses         76       78       +2     

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