Fix neighbour_vehicles not detecting vehicles on connected lanes

[Lidang-Jiang]

Summary

Road.neighbour_vehicles() only searched the current lane segment for front/rear neighbours. Vehicles that had crossed into a connected next or previous lane segment were invisible, causing incorrect behaviour in multi-segment environments like Racetrack, Roundabout, and Intersection.

This PR extends the search to connected lanes:

• Downstream (next segments): found via graph[_to] using the current lane's endpoint as the new start node, with longitudinal offset += current_lane.length
• Upstream (previous segments): found by scanning for graph edges ending at the current lane's start node, with longitudinal offset -= prev_lane.length

All 61 existing tests pass unchanged. 14 new tests added covering same-segment regression, connected-lane detection, curve segments, multi-lane filtering, and edge cases.

Fixes #626

Before (bug reproduction)

BUG CONFIRMED: Front vehicle on connected lane b->c was NOT detected!

Road layout: a->b (50m) connected to b->c (50m)
Ego position: lane a->b, longitudinal=48 (near end)
Front vehicle: lane b->c, longitudinal=5 (on next segment)

neighbour_vehicles result:
  front = None  (WRONG - should detect the vehicle on b->c)
  rear = None

After (bug fixed)

FIXED: Front vehicle detected at [55.  0.]

Road layout: a->b (50m) connected to b->c (50m)
Ego position: lane a->b, longitudinal=48 (near end)
Front vehicle: lane b->c, longitudinal=5 (on next segment)

neighbour_vehicles result:
  front = Vehicle at [55.  0.]  (CORRECT)
  rear = None

pytest results: 75 passed, 1 skipped in 1.82s (was 61 passed, 1 skipped)

Full test output (after)

============================= test session starts ==============================
platform linux -- Python 3.12.12, pytest-9.0.2, pluggy-1.6.0

tests/road/test_neighbour_vehicles.py::TestSameSegmentNeighbours::test_front_and_rear_on_same_lane PASSED
tests/road/test_neighbour_vehicles.py::TestSameSegmentNeighbours::test_no_neighbours PASSED
tests/road/test_neighbour_vehicles.py::TestSameSegmentNeighbours::test_only_front_vehicle PASSED
tests/road/test_neighbour_vehicles.py::TestSameSegmentNeighbours::test_only_rear_vehicle PASSED
tests/road/test_neighbour_vehicles.py::TestConnectedLaneNeighbours::test_front_on_next_segment PASSED
tests/road/test_neighbour_vehicles.py::TestConnectedLaneNeighbours::test_rear_on_previous_segment PASSED
tests/road/test_neighbour_vehicles.py::TestConnectedLaneNeighbours::test_front_on_curve_segment PASSED
tests/road/test_neighbour_vehicles.py::TestConnectedLaneNeighbours::test_closer_same_segment_preferred PASSED
tests/road/test_neighbour_vehicles.py::TestConnectedLaneNeighbours::test_both_connected_front_and_rear PASSED
tests/road/test_neighbour_vehicles.py::TestConnectedLaneNeighbours::test_different_lane_id_not_matched PASSED
tests/road/test_neighbour_vehicles.py::TestEdgeCases::test_no_downstream_connection PASSED
tests/road/test_neighbour_vehicles.py::TestEdgeCases::test_no_upstream_connection PASSED
tests/road/test_neighbour_vehicles.py::TestEdgeCases::test_far_vehicle_still_detected PASSED
tests/road/test_neighbour_vehicles.py::TestEdgeCases::test_lane_index_none PASSED

======================== 75 passed, 1 skipped in 1.82s ========================

Test plan

• 4 same-segment regression tests (existing behaviour preserved)
• 6 connected-lane tests (next segment, previous segment, curve, preference, multi-segment, lane filtering)
• 4 edge case tests (no downstream/upstream, far vehicle, None lane_index)
• All 61 original tests pass unchanged