…ehicles with negative transits

The FinalizeAllowedVehicles() function was using abs() on transit values to check
against vehicle capacity, which incorrectly excluded vehicles when:
- CARGO_LOAD dimensions use negative constants for cumulative load tracking
- Reload dimensions use negative values at landfills/depots

This fix modifies the capacity check to only apply to positive transit values,
since negative transits are used for algorithmic purposes (reload semantics,
load tracking) rather than actual cargo capacity requirements.

Changes:
- Lines 1414-1421: Pre-processing loop now only considers positive transits
  for max_node_transit calculation
- Lines 1444-1448: Main capacity check now skips negative transits entirely

This resolves the regression introduced in commit 847cfc9 (Dec 1, 2023)
affecting OR-Tools versions 9.9.3963 through 9.14.6206.

Fixes: google#4133
Testing: test_ortools_version_standalone_codex.py (multi-class VRP with load tracking)

…llowedVehicles

The FinalizeAllowedVehicles() optimization incorrectly excluded vehicles
from nodes with negative unary transits. The bug checked abs(transit)
against capacity alone, but negative transits are feasible when
abs(transit) <= capacity + slack_max (vehicle can absorb the negative
transit via both capacity headroom and dimension slack).

This caused heterogeneous VRP problems with reload/load-tracking
dimensions to incorrectly exclude smaller vehicles, forcing suboptimal
solutions with dropped stops.

The fix properly bounds negative transits by combining capacity and
slack_max, while positive transits continue to check capacity alone.

Fixes google#4133