MLECO-6773: Refactor config getters/setters to use enum class
- Improve LlmConfig getter/setter type safety by replacing string keys with a strongly typed enum class
- Replace #pragma once with include guards in LlmFactory.hpp for consistency
Change-Id: I0b4b32a77fde031a39e9cca7355e054b61e2752e Signed-off-by: Yunus Kalkan yunus.kalkan@arm.com