Add log_posterior() and nuts_params() methods for draws objects

[utkarshpawade]

Fixes #540

• Adds log_posterior() and nuts_params() methods for draws_array, draws_df, and draws_matrix objects from the posterior package.
• Refactors existing CmdStanMCMC methods to delegate to the new draws_array methods (no behavior change).

[Copilot]

Pull request overview

Adds support for extracting log-posterior values and NUTS sampler diagnostics directly from posterior::draws_* objects, aligning extractor behavior with existing bayesplot plotting helpers that already accept draws objects.

Changes:

• Added S3 methods log_posterior() and nuts_params() for draws_array, draws_df, and draws_matrix.
• Refactored CmdStanMCMC extractor methods to delegate to the new draws_array implementations.
• Added tests, documentation aliases/usages, and a NEWS entry for the new extractor support.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
R/bayesplot-extractors.R	Implements new draws_* S3 methods and delegates CmdStanMCMC methods to them.
man/bayesplot-extractors.Rd	Documents new S3 method aliases/usages for draws_* extractors.
NAMESPACE	Registers the new S3 methods for draws_array, draws_df, and draws_matrix.
tests/testthat/test-extractors.R	Adds coverage for structure + error behavior of new draws-object extractors.
NEWS.md	Announces new extractor methods for posterior::draws_* objects.

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 95.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 99.14%. Comparing base (44c3e5c) to head (56abf4b).

Files with missing lines	Patch %	Lines
R/bayesplot-extractors.R	95.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #541      +/-   ##
==========================================
- Coverage   99.15%   99.14%   -0.02%     
==========================================
  Files          35       35              
  Lines        5920     5935      +15     
==========================================
+ Hits         5870     5884      +14     
- Misses         50       51       +1     

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[jgabry]

Thanks for working on this, but I'm not sure it's needed. After thinking about this, I don't think these methods are the right fit for draws objects. The log_posterior() and nuts_params() extractors are designed to pull specific quantities out of opaque fitted model objects. In practice, if you have a draws object from cmdstanr (via fit$draws()), then you also have the CmdStanMCMC fit object, so you can just use nuts_params(fit) and log_posterior(fit) directly. I'm going to close this along with #540.