Detach generate_plugin_args() output from its testServer session

[nbenn]

Summary

• shiny's upcoming module-scoped reactive cleanup (rstudio/shiny#4372) tears down session-scoped reactives when testServer() closes. generate_plugin_args() returned the board's live reactiveValues (and the nested block reactives), so callers that read them after the helper returns will hit "its module session has been destroyed."
• Re-create the returned reactive parts on a NULL reactive domain (new internal snapshot_reactives()), so they outlive the setup session — the issue's snapshot suggestion combined with its withReactiveDomain(NULL, …) mechanism, applied in the helper rather than in the production board_server().
• Keep types intact (reactiveValues stays reactiveValues, not plain data): blockr.dock reads server$cond via reactiveValuesToList(), which the literal isolate(reactiveValuesToList(rv)) suggestion would break.
• Defer errors from not-yet-evaluable reactives (e.g. a block with unset inputs) to read time, matching the lazy behaviour of the live reactive — eager evaluation during the snapshot would otherwise fail the whole call.

I also ran the downstream consumers of this helper (blockr.code, blockr.dock's board_args tests) against the patched core; both stay green.

Fixes #196

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Files with missing lines	Coverage Δ
R/utils-tests.R	94.91% <100.00%> (+1.43%)	⬆️

... and 3 files with indirect coverage changes

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