Fix #1582: Include custom base model fields in diff_against()

[ace2016]

[Improvement] – Include custom base model fields in diff_against() deltas

Description

Context:
Previously, fields added via a custom base class (passed using the bases parameter of HistoricalRecords) were excluded from the results of diff_against(). This happened because diff_against() used tracked_fields as its default set of included fields, and tracked_fields is explicitly designed by the library to only include the fields present on the original model. As a result, custom base fields (like custom session tracking or IP address tracking columns) would silently be omitted from history diffs, even when their values clearly changed between versions (Issue #1582).

Approach:

• Modified the default included_fields logic within HistoricalChanges.diff_against() in simple_history/models.py.
• Instead of strictly falling back to tracked_fields, the method now cleanly scans the historical model's _meta.fields for any editable fields that are not already tracked and are not internal history metadata (e.g., history_id, history_date, history_type, etc.).
• Added a targeted regression test (test_history_diff_includes_custom_base_fields) in simple_history/tests/tests/test_models.py leveraging the existing PollWithHistoricalSessionAttr test model.

Impact:

• Functionality: diff_against() now correctly identifies and includes changes to custom base fields (e.g., session or ip_address fields added via bases) in its returned ModelDelta objects.
• Developer Experience: Resolves a confusing pain point where custom tracked fields were successfully stored in the database but wouldn't appear in diff comparisons.
• Robustness: The fix strictly avoids modifying the generic tracked_fields definition, ensuring that other features reliant on tracked_fields (like most_recent() and bulk_history_create()) remain completely unaffected and safe.

Visual Proof / Evidence

The entire test suite ran successfully, including the new regression test, proving the fix works precisely as intended and maintains full backwards compatibility with all existing diff behaviors.

Tests

• New tests added
• Existing tests updated or refactored

Unit Tests Summary:

• Added test_history_diff_includes_custom_base_fields in simple_history/tests/tests/test_models.py: Leverages the PollWithHistoricalSessionAttr model to create snapshots with different custom field values (session-old and session-new). Calling diff_against() asserts that the session field name safely populates into delta.changed_fields, and validates that its old and new values correctly appear inside delta.changes.

[ace2016]

Hello @ddabble This PR addresses issue #1582 by ensuring custom base model fields are properly included in diff_against() deltas. Could you please take a look when you have a chance? All tests are passing. Thanks!