feat: pre-fill create form with SQLAlchemy column defaults (#690)

[SHYXIN]

Summary

When rendering the create form, extract default values from the SQLAlchemy model's column definitions and pass them as the initial obj to the template. This pre-fills form fields with their database-defined defaults.

Changes

• starlette_admin/base.py — Added _get_default_values() method that reads column.default.arg from the model's __table__
• _render_create() — On GET requests, calls _get_default_values() and passes the result as obj to the template context

How it works

For SQLAlchemy models, the method iterates over model.__table__.columns and extracts col.default.arg for columns that have a Python-side default. For non-SQLAlchemy models, returns an empty dict (no change in behavior).

Example: a column defined as status = Column(String(20), default='active') will pre-fill the form field with 'active'.

Testing

• All 14 views tests pass
• Manual verification with SQLAlchemy model defaults

Additional

Includes chore commit to fix pyproject.toml pytest config conflict.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (0f684fe) to head (19a2ece).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #771   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           86        86           
  Lines         6848      6860   +12     
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+ Hits          6848      6860   +12     

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