Add test for BridgeGrillageModel analyser

[ManavSharma23]

Summary

This PR replaces the placeholder analyser test with a detailed unit test suite for BridgeGrillageModel. The tests verify how the plate girder bridge analyser initializes state, builds geometry/material/member definitions, creates grillage models, applies loads, runs analysis, and prepares result/plot workflows.

Details

The new tests use pytest fixtures and mocks to isolate analyser logic from external OpenSees/grillage execution. They check that:

• bridge geometry and deck layout inputs are stored and passed correctly
• section, material, and member objects are created with expected properties
• grillage model creation assigns the correct beam/slab members, including edge beam behavior
• dead loads are created for girder self weight, deck slab, wearing course, footpath, crash barrier, railing, and median components
• dead load combinations include available sub-load cases with the expected load factor
• wind, live load, static vehicle, and moving vehicle load cases are generated and registered correctly
• IRC vehicle lane placement, impact factor/DLA handling, and vehicle type mapping behave as expected
• analysis, result handling, governing live load selection, and plotting methods call the expected model/result APIs

Testing

Adds 108 unit tests in tests/unit/test_analyser.py.