Fix service rate editor persistence for saved fees

[roncodes]

Summary

• fix service rate editor round-trip behavior for per-drop and related fee rows
• preserve saved fee records in the serializer/model layer instead of collapsing rows incorrectly on reload
• add targeted unit coverage for the editor-facing model behavior

What Changed

• updated the serializer fee merge logic to use stable keys for persisted and per-drop records
• updated the service-rate model to expose per-drop fees from rate_fees using waypoint-based rows sorted by minimum threshold
• preserved the existing fixed-distance filtering behavior for non per-drop calculation methods
• added model tests covering per-drop ordering and fixed-distance filtering

Verification

• added unit coverage in tests/unit/models/service-rate-test.js

Notes

• this PR is intentionally limited to the data/model layer used by the FleetOps editor
• style-only lint/prettier cleanup was intentionally left out of scope for this PR