Fixes #17954 - Add dynamic parent resolution for cable CSV imports

[jsenecal]

Fixes: #17954

Overview

This PR implements dynamic parent resolution for cable CSV imports, enabling bulk import of cables connected to circuits, power panels, and devices. This is a revised approach following PR #17995, which was closed due to design concerns.

Key Difference from PR #17995:

Instead of adding separate fields for each parent type (side_x_circuit, side_x_power_panel), this implementation uses dynamic parent resolution by repurposing the existing side_x_device field to side_x_parent. The parent object (Circuit, Device, or PowerPanel) is determined based on the termination type specified.

Implementation Details

1. Dynamic Parent Field Resolution:

• Renamed side_x_device to side_x_parent in CableImportForm
• The parent field now dynamically resolves to the appropriate model based on side_x_termination_type
• Supports three parent types:
  • dcim.device → Resolves using Device model
  • circuits.circuit → Resolves using Circuit model
  • dcim.powerpanel → Resolves using PowerPanel model

2. Parent-Specific Lookup Logic:

• Circuits: Uses cid (Circuit ID) as the lookup field
• Devices: Uses name as the lookup field
• Power Panels: Uses name as the lookup field
• Each parent type uses its appropriate queryset and to_field_name

3. Enhanced Validation:

• Validates that circuit terminations are not already connected to Provider Networks
• Ensures proper parent object resolution based on termination type
• Maintains backward compatibility with existing device-only imports

4. Comprehensive Test Coverage:

• Added/Modified test cases to CableTestCase for new field names and supported variations
• Added validation tests in CableImportFormTestCase for code that CableTestCase doesn't reach (all error paths)
• Tests cover all three parent types (circuits, devices, power panels) and validation tests for edge cases and error conditions

Benefits

• Single Field Design: Cleaner CSV format - one parent field per side instead of multiple conditional fields
• Extensible: Easy to add support for additional parent types in the future
• Backward Compatible: Existing device-based imports continue to work without changes
• Comprehensive: Solves both circuit terminations AND power feeds as suggested in PR Fixes #17954 - Handle CircuitTerminations in Cable Bulk Import #17995

Usage Example

side_a_termination_type,side_a_parent,side_a_termination,side_b_termination_type,side_b_parent,side_b_termination,type,status
circuits.circuittermination,CID123,A,dcim.interface,Device1,eth0,cat6,connected
dcim.powerport,Device2,PSU1,dcim.powerfeedtermination,PowerPanel1,1,power,connected