Add deserialization_exception field for programmatic error handling

[bjester]

Summary

This allows programmatic handling of deserialization errors instead of parsing error messages

• Adds a new deserialization_exception nullable field to Store model that stores the fully-qualified exception class path when deserialization fails
• Updates existing deserialization_error field to be nullable
• Updates handling of deserialization errors to set both fields, or otherwise null both of them
• Adds tests for changes
• Removes migration tests that verified the squash migration, which now cause issues with new migrations

References

Followup to #306
Closes #315

AI Usage

Directed an AI agent to tackle the issue, iterated on its work for a loop, then did a small refactor myself. Had the AI agent add test coverage, then reviewed.