Make sure metadata() has correct value for num_chains

[jgabry]

Submission Checklist

• Run unit tests
• Declare copyright holder and agree to license (see below)

Summary

fixes #1186
Fixes num_chains in fit$metadata(). Previously it was always returning 1. I suppose it should really be chains instead of num_chains to match CmdStanR's arguments, but since we've been using num_chains (inherited from the CmdStan CSV names) I'll leave it as num_chains for backwards compatibility.

Copyright and Licensing

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Jonah Gabry

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license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.19%. Comparing base (48b825d) to head (3b3fbc6).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1187      +/-   ##
==========================================
- Coverage   91.21%   91.19%   -0.02%     
==========================================
  Files          15       15              
  Lines        6121     6121              
==========================================
- Hits         5583     5582       -1     
- Misses        538      539       +1     

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[jgabry]

@VisruthSK I think this windows run is failing at the spot that you changed in #1182

https://github.com/stan-dev/cmdstanr/actions/runs/25310119846/job/74194792020?pr=1187#step:13:7723

Seems like it's not deterministic. I think it also initially failed in your roxygen2 PR but then passed when I reran it (and it failed again after I merged it into master). Rerunning again now to see if it passes. We also have some non-determinism in the Mac tests that I'd like to sort out at some point (if possible). I tried at one point, but then got distracted with other work. I can try again at some point, or if you're interested you can take a look. Not super urgent.