#9609 packaging: Remove unmaintained dependency `flatten-dict` Pull request opened by edgarrmondragon ## Description ## Related Issues • Closes #XXXX ## Summary by Sourcery Remove the unmaintained flatten-dict dependency by providing in-house flatten and unflatten utility functions, updating all usages and tests accordingly. Enhancements: • Implement a custom flatten function with "dot", "env_var", and "tuple" reducers and type overloads • Add an unflatten function to reconstruct nested dictionaries from flat key mappings • Replace all external flatten_dict calls in settings_store and manifest with the new implementations Build: • Remove flatten-dict from dependencies and update lock files Tests: • Extend flatten tests to cover dot, env_var, and tuple reducers • Add comprehensive unflatten tests, including round-trip and custom splitter scenarios meltano/meltano