fix(fastapi): resolve session lifecycle timing with generator dependencies

[cofin]

Summary

Fixes #647

This PR resolves the session lifecycle timing conflict between Starlette middleware and FastAPI's generator-based dependency cleanup (provide_service()). The issue was causing asyncpg connections to not be properly returned to the pool, resulting in:

• InvalidRequestError: An invalid request was made.
• SAWarning: The garbage collector is trying to clean up non-checked-in connection...

Root Cause

The middleware was closing sessions before FastAPI's generator dependency cleanup could run:

1. Request arrives
2. FastAPI resolves `provide_service()` dependency (generator)
3. Session created and stored in `request.state`
4. Generator yields service, suspends
5. Route handler executes
6. Response is generated
7. Middleware runs → session_handler() CLOSES SESSION ← Problem
8. FastAPI dependency cleanup resumes generator
9. Generator's `async with` tries to exit with closed session
10. asyncpg connection not properly returned → GC warning

Solution

• Mark sessions as "generator-managed" when using provide_service()
• Middleware stores response status but skips cleanup for generator-managed sessions
• Generator handles commit/rollback/close in its finally block with access to the stored response status

Changes

File	Changes
advanced_alchemy/extensions/fastapi/providers.py	Add _should_commit_for_status() helper, update provide_service() to handle lifecycle
advanced_alchemy/extensions/starlette/config.py	Update middleware dispatch to skip generator-managed sessions
tests/unit/test_extensions/test_fastapi/test_session_lifecycle.py	50 new tests covering session lifecycle scenarios

Test Plan

• All 176 FastAPI unit tests pass
• 50 new session lifecycle tests covering:
  • _should_commit_for_status() helper function
  • Commit strategies with autocommit, autocommit_include_redirect, and manual modes
  • Response status codes (2xx, 3xx, 4xx, 5xx)
  • Middleware skipping generator-managed sessions
  • Non-generator sessions still use middleware cleanup
  • Multiple configs with different session keys
• Linting passes

[codecov-commenter]

Codecov Report

❌ Patch coverage is 80.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.45%. Comparing base (947df81) to head (524dcdc).

Files with missing lines	Patch %	Lines
advanced_alchemy/extensions/fastapi/providers.py	80.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #648      +/-   ##
==========================================
+ Coverage   80.44%   80.45%   +0.01%     
==========================================
  Files          87       87              
  Lines        6453     6462       +9     
  Branches      838      841       +3     
==========================================
+ Hits         5191     5199       +8     
- Misses        998      999       +1     
  Partials      264      264              

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