Improvements to RRC PDU session and DRB handling logic
This merge request introduces a major simplification and cleanup of the gNB RRC
PDU Sessions and DRBs handling logic. The core goals of this work are to:
- Replace fixed-size PDU Session, DRBs and QoS arrays with dynamic containers
- Simplify and modularize helper functions for session/DRB management
- Prepare for further refactoring of the PDU Session state machine
- Improve code safety and maintainability through better bounds checking