normalizeConfigThinkingLevel function
Normalizes a config-declared level to its ladder rung: known rungs pass
through clampThinkingLevel (xhigh/max fold to high), anything
else returns null for the caller to reject with its own named error.
Implementation
String? normalizeConfigThinkingLevel(String level) =>
configThinkingLevels.contains(level) ? clampThinkingLevel(level) : null;