Fix per-girder stiffener preservation and render longitudinal stiffeners on both sides

[MdTalha17]

Description

This PR resolves issues with per-girder stiffeners and girder properties being globally reset to defaults during solve execution, and fixes the 3D CAD representation of longitudinal stiffeners.

Key Fixes:

1. Per-Girder Configuration Preservation: Prevented solve_extend_basic_input_dict() from deleting and overwriting user-configured inputs in defaults.py on subsequent design solver runs.
2. Design Mode Switch Safety Check: Handled transition from Optimized to Custom design mode by overwriting "All", None, or empty values with solved defaults, avoiding conversion/multiplication TypeError exceptions.
3. CAD Parameter Binding: Passed stiffeners_dict down to BridgeParametersDTO so the CAD generator actually receives and renders the girder-specific stiffeners configurations.
4. Symmetric Longitudinal Stiffeners: Updated the 3D girder geometry builder to construct longitudinal stiffeners on both sides of the girder web (left and right), rather than just on one side.

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Detailed Changes

1. Core Bridge Defaults Extension

• File: src/osdagbridge/core/bridge_types/plate_girder/defaults.py
  • Updated _on_no_of_girders_changed() to check girder indices via regex (e.g., matching .G{i}).
  • Only dynamic keys corresponding to girder indices exceeding the new girder count (or bracing index exceeding no_of_bracings) are deleted as stale, preserving valid configurations.
  • Added safety checks (key not in working_input_dict) to prevent resetting existing user inputs to global defaults.
  • Added fallback overwrite for "All", None, or empty string "" girder dimensions in Custom design mode to prevent solver crash errors.

2. CAD Parameters DTO Mapping

• File: src/osdagbridge/core/bridge_types/plate_girder/plategirderbridge.py
  • Passed the parsed stiffeners_dict into BridgeParametersDTO inside get_3d_cad_parameters(), allowing the CAD generator to obtain per-girder configurations.

3. Girder Geometry Builder

• File: src/osdagbridge/core/bridge_components/super_structure/plate_girder/builder.py
  • Modified build_plate_girder_geometry() within the longitudinal stiffeners block to construct and append plates on both sides of the web:
    • Right side: Centered at seg.tw / 2 + long_stiff_width / 2
    • Left side: Centered at -seg.tw / 2 - long_stiff_width / 2

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