Skip virtual fields like GenericForeignKey

[craigds]

Fix AttributeError when models have GenericForeignKey fields.

Virtual fields (concrete=False) like GenericForeignKey don't have database columns, causing AttributeError when accessing field.attname. This PR skips them like reverse relations are skipped.

Fixes #16

[Copilot]

Pull request overview

This PR fixes an AttributeError that occurs when Django models contain GenericForeignKey or other virtual fields (fields with concrete=False). Virtual fields don't have database columns and lack the attname attribute, causing crashes when the library attempts to track field changes. The fix filters out virtual fields automatically and raises a clear error when they're explicitly requested.

• Adds is_virtual_field() helper to identify virtual fields by checking the concrete attribute
• Filters out virtual fields during auto-detection (similar to existing reverse relation filtering)
• Adds runtime check to skip virtual fields in get_and_update_changed_fields()
• Provides clear error messages when virtual fields are explicitly requested
• Adds comprehensive test coverage for virtual field handling

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
fieldsignals/signals.py	Implements virtual field detection and filtering logic to prevent AttributeError when accessing field.attname on GenericForeignKey and similar virtual fields
fieldsignals/tests/test_signals.py	Adds VirtualField mock class, FakeModelWithVirtualField test model, and test cases to verify virtual fields are auto-excluded and raise clear errors when explicitly requested

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