Take over PR #3888: Add unit tests for benchmarking metrics and document metric_R2

[ayushman1210]

I am taking over #3888 PR because the original author has been inactive, and I didn't have the permissions to push directly to their fork.
This PR includes all of @tanmaydimriGSOC's original commits, plus the requested fixes to unblock it

• Removed the PEcAn.logger mock from the top of test-metrics.R.
• Refactored metric_R2.R to remove the unreliable lm() fallback, now using stats::cor()^2.
• Tightened the unit tests for metric_RMSE (expecting exactly 0) and metric_R2 (expecting NA and the standard stats::cor warning for constant inputs).

This can be merged to fully close out #3888

[divine7022]

per yesterday's discussion, please try pushing to 3888, since you haven't actually tried
don't really want to have diverging PRs on same

[divine7022]

LGTM overall, and thanks for picking this up cleanly. tanmay's commits are preserved. dropped inline, few things before we commit
re run the devtools::document('modules/benchmark') to regenerate docs it is not in synce

[divine7022]

ci is failing, please regenerate docs by running devtools::document() from package root and run Rscript scripts/generate_dependencies.R generate docker depends

[dlebauer]

@ayushman1210 can you please

git checkout pr_3888 # the branch this PR is from
make document
git status
git add models/sipnet/man/read_restart.SIPNET.Rd
git commit -m "Regenerate SIPNET documentation"
git push

[dlebauer]

Thanks for adding tests! Ready to merge when CI passes (see prev. comment about regenerating docs)

[ayushman1210]

@dlebauer Commit f14de15b6 ("style: regenerate package documentation") has already regenerated the documentation

[dlebauer]

@ayushman1210 have you incorporated all comments from #3888? If so, can 3888 be closed?

[ayushman1210]

Yes @dlebauer I’ve incorporated all the comments from PR #3888.
You can close it now
thanks !!

[divine7022]

thanks!, looks good now. all CI checks are passing
good to merge