Fix dynamic field tracking

[ace2016]

[Improvement] – Fix historical tracking for dynamically added model fields

Description

Context:
Prior to this change, django-simple-history suffered from a timing limitation where dynamically created fields were not being tracked. Historical models are generated by copying the concrete model's fields during the Django class_prepared signal. However, custom metaclasses (such as django-organizations' OrgMeta) often add fields (like ForeignKey relationships) to models after the class_prepared hook has fired. Consequently, these late-added fields were entirely omitted from the generated historical tracking models, breaking history functionality for those fields.

Approach:
This fix introduces a hybrid re-finalization pattern to ensure all fields are captured without disrupting the framework's startup timing dependency on settings:

• Immediate Finalization with Tracking: Modified the existing HistoricalRecords.finalize() method to continue building historical models immediately upon class_prepared, preserving reliance on module-level settings (like SIMPLE_HISTORY_HISTORY_ID_USE_UUID). However, it now records a field count snapshot in _finalized_models.
• Deferred Re-evaluation in AppConfig: Created SimpleHistoryAppConfig with a ready() hook that calls a new function, _process_pending_finalizations().
• Dynamic Field Detection & Re-creation: During ready(), the framework iterates over _finalized_models. If a model's current field count differs from its snapshot (indicating dynamically added fields), its historical model is safely re-created to incorporate the new fields.
• Signal Cleanup: Implemented logic within _process_pending_finalizations() to explicitly disconnect outdated post_save, post_delete, and pre_delete signals before re-finalization, explicitly preventing duplicate tracking records.

Impact:

• Functionality: Dynamically added fields (specifically those added via metaclasses post-class_prepared) are now correctly included and tracked in historical models.
• Robustness: Mitigates the core issue (Issue Dynamically created fields aren't tracked #1517) comprehensively while avoiding regressions related to runtime feature flags and settings.
• Coverage: Provides complete end-to-end verification through a custom metaclass test suite, bringing total tests passing to 365 (up from 360).

Visual Proof / Evidence

Tests

• New tests added
• Existing tests updated or refactored

Unit Tests Summary:

Created the DynamicFieldTrackingTest class containing 5 new end-to-end tests:

• test_historical_model_has_dynamic_field: Verifies the dynamic group ForeignKey exists on the reconstructed historical model.
• test_historical_model_for_group_exists: Ensures the base DynamicGroup model tracks standard fields as expected.
• test_no_duplicate_history_on_save: Confirms exactly 1 history record generated upon model creation (verifying valid signal cleanup).
• test_no_duplicate_history_on_update: Validates that subsequent updates generate exactly 1 additional tracking delta.
• test_dynamic_field_value_tracked_in_history: Manually tests value mutation over time to ensure dynamic ForeignKey values persist accurately across the history tables.