fix: read mapping, filters, ratio_table, and units from YAML in get_settings_code()

[Gero1999]

Issue

Closes #1189

Description

get_settings_code() used a hardcoded default_mapping parameter and ignored filters, ratio_table, units_table, and extra_vars_to_keep stored in the settings YAML. Generated R scripts would use wrong mapping and skip filters, ratios, and unit overrides.

Changes:

• Read mapping, filters, ratio_table, units_table from the YAML when present
• Derive extra_vars_to_keep from mapping$select_Grouping_Variables (consistent with the UI path in nca_results.R)
• Read time_duplicate_keys from the YAML instead of hardcoding NULL
• Rename default_mapping to .default_mapping (internal, not exported) and use it as fallback for legacy YAML files
• Remove the mapping parameter from the public API

Definition of Done

• get_settings_code() reads mapping from YAML, falls back to .default_mapping for legacy files
• get_settings_code() includes applied_filters from YAML in the session object
• get_settings_code() includes ratio_table from YAML in the session object
• get_settings_code() includes units_table from YAML in the session object
• get_settings_code() derives extra_vars_to_keep from mapping
• Generated R scripts correctly apply mapping, filters, units, and ratio calculations

How to test

1. Generate a settings YAML from the app (with custom mapping, filters, ratio table, and units)
2. Call get_settings_code(settings_file_path, data_path) — no mapping argument needed
3. Inspect the generated R script: it should contain the custom mapping values, filters, ratio table, and units from the YAML
4. Test with a legacy YAML (no mapping key) — should fall back to CDISC defaults

Contributor checklist

• Code passes lintr checks
• Code passes all unit tests
• New logic covered by unit tests
• New logic is documented
• App or package changes are reflected in NEWS
• Package version is incremented
• R script works with the new implementation (if applicable)
• Settings upload works with the new implementation (if applicable)
• If any .scss change was done, run data-raw/compile_css.R

Notes to reviewer

• This is a breaking change for anyone calling get_settings_code(mapping = ...) explicitly. The mapping parameter has been removed since the YAML already contains the mapping (since Enhancement: Add mapping used during session to settings #721). Legacy YAML files without a mapping key still work via .default_mapping fallback.
• R is not available in the dev container, so tests could not be run locally. CI will validate.
• Related to Enhancement: Settings Features #718 (parent), Enhancement: Derive extra_vars_to_keep consistently from mapping across UI, R script, and get_settings_code() #1187 (extra_vars_to_keep), Bug: units_table path mismatch between settings YAML export and R script template #1188 (units_table path). This PR addresses the get_settings_code() side; the script template side (Enhancement: Derive extra_vars_to_keep consistently from mapping across UI, R script, and get_settings_code() #1187) and units format (Bug: units_table path mismatch between settings YAML export and R script template #1188) may need separate follow-ups.