Improve test suite coverage and validation

[ThomUK]

Summary

• Add edge case and boundary tests for get_target_text, get_assurance_type, get_variation_type, parse_rebase_dates, and align_rebase_dates
• Add medium-priority tests for check_a_data, check_measure_config, check_dataset_is_complete, process_event_data_t, and lengthen_measure_data
• Add check_for_allowed_values() helper and apply it to validate improvement_direction, unit, spc_chart_type, and aggregation against their allowed sets (with tolower normalisation)
• Move "no date columns" guard upstream into check_a_data() so it fires at bundle entry rather than deep in lengthen_measure_data()
• Fix copy-paste bug: check_a_data() error message incorrectly said "check_measure_data:"
• Remove now-redundant assert_that(!any(is.na(aggregation))) check in check_report_config() — covered by check_for_allowed_values
• Convert all test files from "description" |> test_that({}) pipe style to test_that("description", {}) for consistency

Test plan

• devtools::test() passes with 0 failures

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