Refactor AqueousEquilibrium in musica.mechanism_configuration to use composition instead of inheritance from _AqueousEquilibrium.

Acceptance Criteria

• The class no longer inherits from _AqueousEquilibrium.
• The internal C++ instance is held in self._instance.
• All properties (name, aerosol_phase, aerosol_phase_water, reactants, products, A, C, k_reverse, other_properties) delegate to or update self._instance.
• Reactants and products use Python Species and Phase objects on the Python side and convert to/from C++ _ReactionComponent.
• The serialize() method is converted from a static method to an instance method, operating solely on Python-visible data.
• Public interface exposes only Python-native types, no direct C++ inheritance.

Ideas

• Use the tuvx.py wrapper pattern as a model for composition.
• Implement helper functions to convert between Species/Phase and _ReactionComponent.
• Use @property decorators for clean delegation of attributes.
• Refactor serialize() to eliminate static method usage.
• Do the work similar to Refactor CondensedPhaseArrhenius to use composition instead of inheritance #461 Refactor Arrhenius class to use composition instead of inheritance #462, Refactor Troe class from inheritance to composition pattern #466, Refactor Branched class to use composition instead of inheritance #467

Tasks

• Remove inheritance from _AqueousEquilibrium.
• Add self._instance initialized to _AqueousEquilibrium() inside __init__.
• Forward all attribute access and mutations to self._instance via properties.
• Convert reactants and products to Python objects on get, convert to C++ objects on set.
• Change serialize() to an instance method accessing Python-side data.
• Remove static serialize() method and update callers accordingly.