Refactor/perf batch 2

[timcadman]

Summary of changes:

Client function refactoring

22 client functions refactored (ds.abs, ds.asCharacter, ds.asDataMatrix, ds.asInteger, ds.asList, ds.asLogical, ds.asMatrix, ds.asNumeric, ds.class, ds.colnames, ds.completeCases, ds.dim, ds.exp, ds.isNA, ds.length, ds.levels, ds.log, ds.ls, ds.names, ds.numNA, ds.sqrt, ds.unique):

• Replaced datasource check and defaults boilerplate with .set_datasources()
• Removed clientside validation of existence and class — validation now handled server-side by.loadServersideObject() and .checkClass()
• Removed MODULE 5 object-creation checks (isAssigned, messageDS, is.object.created/validity.check returns)
• Functions that call dimDS, lengthDS, numNaDS, isNaDS now unwrap list returns ($dim, $length, $numNA, $is.na)

New shared utilities in R/utils.R: .set_datasources(), .verify_datasources(), .checkClassConsistency(),
.check_df_name_provided()

Downstream fixes

13 additional client functions updated because they call refactored server functions: glmChecks, ds.summary, ds.quantileMean, ds.recodeLevels, ds.replaceNA, ds.rowColCalc, ds.subsetByClass, ds.dataFrameFill, meanByClassHelper0b, meanByClassHelper2, meanByClassHelper3, subsetHelper, checkClass. Key changes:

• as.symbol(paste0("functionDS(...)")) → call("functionDS", ...) where server functions now expect string names
• Unwrap new list return formats from dimDS, lengthDS, numNaDS
• Handle .loadServersideObject() erroring on missing columns (previously returned NULL silently)

Test updates

• Wrong-class checks moved from arg to smk tests (now server-side errors)
• $ValidityMessage expectations removed (levelsDS no longer returns it)
• DISCORDANT tests updated for server-side column-not-found errors
• Performance tests added for refactored functions