Add functions to manage UE serving cells so the RRC can track which cells
a UE is using (PCell and SCells) for multi-cell UE context and future
SCell management.
- rrc_cell_management.c / rrc_cell_management.h:
* rrc_get_ue_serving_cell_by_id(): return serving cell by servCellIndex
(O(k) linear search, k ≤ 32).
* rrc_add_ue_serving_cell(ue, cell, serving_cell_id): add entry to UE
serving_cells with validation: reject duplicate servCellIndex with
different nci, enforce max 32 serving cells (TS 38.331), keep PCell
at index 0 for O(1) access; return existing entry if same
servCellIndex and nci.
* rrc_remove_ue_scells_from_du(ue, assoc_id): remove all serving cells
for the given DU (handover/DU disconnect); use stored assoc_id for
O(k) comparison.
* ue_get_pcell_entry(): return PCell entry (first element, O(1)).
* rrc_get_pcell_for_ue(): return PCell container from cell tree using
UE's PCell entry.
* Header: RRC_PCELL_INDEX (0), RRC_MAX_NUM_SERVING_CELLS (32), and
declarations for the above.
- UE context lifecycle:
* rrc_gNB_create_ue_context(): seq_arr_init(serving_cells).
* rrc_delete_ue_data(): seq_arr_free(serving_cells).