use SQLModel's AsyncSession when available, fallback to SQLAlchemy's

[h0rn3t]

Resolving AsyncSession's 'exec' AttributeError in FastAPI with SQLModel

[trunet]

It's working for me with this PR.

[codecov]

Codecov Report

❌ Patch coverage is 71.42857% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.45%. Comparing base (15a798b) to head (579f607).
⚠️ Report is 24 commits behind head on main.

Files with missing lines	Patch %	Lines
fastapi_async_sqlalchemy/middleware.py	69.23%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #28      +/-   ##
==========================================
- Coverage   97.22%   92.45%   -4.77%     
==========================================
  Files           3        3              
  Lines          72      106      +34     
==========================================
+ Hits           70       98      +28     
- Misses          2        8       +6     

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