feat: adding timestamp for sessions

[felipemontoya]

Simple PR to add timestamps for ai_sessions. This is very helpful when trying to review or debug the inner workings of the AI from the Django admin.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.33%. Comparing base (e962171) to head (61bd852).

Additional details and impacted files

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##             main     #221   +/-   ##
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  Coverage   95.33%   95.33%           
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  Files          69       69           
  Lines        8073     8075    +2     
  Branches      429      429           
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+ Hits         7696     7698    +2     
  Misses        283      283           
  Partials       94       94           

Flag	Coverage Δ
unittests	95.33% <ø> (+<0.01%)	⬆️

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[Copilot]

Pull request overview

Adds created_at and updated_at timestamp fields to the AIWorkflowSession model to aid debugging and review via the Django admin.

Changes:

• Add created_at (auto_now_add) and updated_at (auto_now) fields to AIWorkflowSession.
• Add migration 0008 to introduce the two nullable timestamp columns.
• Surface the new timestamps in the Django admin via list_display, list_filter, date_hierarchy, and readonly_fields.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
backend/openedx_ai_extensions/workflows/models.py	Adds the two new timestamp fields to the model.
backend/openedx_ai_extensions/migrations/0008_aiworkflowsession_timestamps.py	New migration adding the columns as nullable.
backend/openedx_ai_extensions/admin.py	Displays/filters the new timestamps in the session admin.

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[felipemontoya]

This is a straight forward change. Merging after there was not even comment by copilot