feat(uncertainty): outdir opt-in for get.parameter.samples wrapper (Modularity Part 3)

[omkarrr2533]

Third PR in the trait → meta-analysis → parameter-sampling modularity series (after #3860 Part 1, #4010 Part 2). Cleans up the get.parameter.samples() wrapper now that the pure get_parameter_samples() from #3860 does the computation.

What changed

• Replaces the save_to_disk flag (added in feat: extract pure parameter sampling logic + tests (Modularity Part 1) #3860) with an outdir parameter, default settings$outdir. samples.Rdata is written when outdir is non-NULL (the default) and skipped when outdir = NULL. This matches the outdir-as-opt-in-provenance convention used across the rest of the refactor.
• The wrapper stays .Deprecated and still delegates to get_parameter_samples(). The return value is unchanged from develop — it returns the samples list invisibly (the switch from invisible(NULL) to the list landed in feat: extract pure parameter sampling logic + tests (Modularity Part 1) #3860); this PR just adds a test that locks that contract in.

Backward compatibility

No behavior change for existing callers — none of them pass the new argument, so outdir defaults to settings$outdir and samples.Rdata is written exactly as before. The only removal is save_to_disk: grepped the repo, it appears only inside get.parameter.samples.R and nothing passes it, so dropping it is safe (it was never in a release).

Per the Slack discussion with @mdietze and @infotroph: nothing changes for disk-writing workflows in this PR. The ensemble.samples.*.Rdata / sensitivity.samples.*.Rdata files that workflows like the SIPNET segmented restart (#3919 / #4008) rely on are written by run.write.configs(), which this PR does not touch, and samples.Rdata still writes by default. Migrating the generate_*_design() and SDA callers to the dash function is deferred to Phase 2/3.

Tests

Adds test-get.parameter.samples.R in the correct package path (modules/uncertainty/): default save to settings$outdir, explicit outdir override, outdir = NULL skip, the 5-object samples.Rdata name contract, invisible return, and delegation to get_parameter_samples(). load.posteriors() and get_parameter_samples() are stubbed with mockery, so it runs with no database. Local run for this file: [ FAIL 0 | SKIP 0 | PASS 10 ].

Also relocates the misplaced test-get.parameter.samples.R under modules/meta.analysis/tests/testthat/ (added in #3858): renamed to test-p.point.in.prior.R since its remaining tests cover the PEcAn.MA helper p.point.in.prior(), and dropped the obsolete get.parameter.samples skip-test (stale since #3860, superseded by the test added here).

[S1DDHEY]

One note on the description: it says the wrapper “previously returned NULL invisibly”, but on current develop get.parameter.samples() already ends with invisible(result) (returns the list invisibly). In this PR, I only see the return behavior being preserved, not changed.

If you meant “it still returns invisibly (not NULL)”, could you please update the PR description accordingly? Or point to the commit/branch state where it was returning NULL so the statement matches history.

Also currently there is another test-get.parameter.samples.R under modules/meta.analysis/tests/testthat/ which firstly is in the wrong place (added in #3858) and also is not needed anymore as you've added one here with the correct path in this PR, would suggest removing it.

[omkarrr2533]

thanks @S1DDHEY.
On the return you're right, that line was misleading. The switch from invisible(NULL) to returning the list happened back in #3860; this PR doesn't change the return, just preserves it and adds a test to lock the contract in. Updated the description.
On the misplaced test looked at it, and that file's a mix: the get.parameter.samples test in there was just a skip() placeholder for the old behaviour, but the same file also holds the p.point.in.prior tests, which are real PEcAn.MA helper tests. So rather than delete it I renamed it to test-p.point.in.prior.R and dropped the obsolete skip-test clears the misplaced get.parameter.samples test without losing the prior coverage.