Fix flaky/fragile tests: add skip guards for optional dependencies and widen timestamp tolerance

[Copilot]

Several unit tests introduced in the previous commit were brittle: they'd fail in environments missing Suggests-only packages (jsonlite, corrr, ranger, vip), on single-core machines, or on slow CI runners.

Changes

• test-agent_helpers.R — Added skip_if_not_installed("jsonlite") to all extract_json_object() tests
• test-feature_selection.R — Added skip_if_not_installed("corrr") to all target_corr_fn tests; added skip_if_not_installed("ranger") + skip_if_not_installed("vip") to vip_rf_fn test
• test-parallel_util.R — Added skip_if(parallel::detectCores() <= 1) to the local_machine cluster test to prevent makeCluster(0) errors on single-core runners
• test-utility.R — Re-created (was accidentally deleted); widened get_timestamp() timing tolerance from 5s → 60s to reduce flakiness on loaded CI

# Example pattern applied throughout
test_that("extract_json_object parses simple JSON", {
  skip_if_not_installed("jsonlite")
  ...
})

test_that("par_start with local_machine creates cluster", {
  skip_if(parallel::detectCores() <= 1, "Requires more than one core")
  ...
})

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