fix(core): reset Reaper state when ryuk is already being removed

[RitiGrover]

Fixes #1125

What was wrong
When ryuk was killed from outside, Docker was already auto-removing it. delete_instance() got a 409 from stop(), but it only suppressed NotFound. The 409 escaped before _container and _instance were reset, so get_instance() kept returning the dead reaper. The same 409 showed up in the atexit hook.

What changed
A 409 from stop() is treated like NotFound. The socket, container and instance are reset in a finally, so a failing stop() can't leave stale state either. This follows the fix suggested in the issue.

How it was tested
Two unit tests with a fake dead container:

• A 409 is swallowed, the state is cleared and get_instance() creates a new reaper.
• Any other APIError is still raised, but the state is cleared.

Both fail on main and pass with the fix. Ruff passes.

This touches the same function as #1124. Whichever lands second will need a small rebase, which I'll do.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.02%. Comparing base (8d81011) to head (69d94af).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1126      +/-   ##
==========================================
- Coverage   86.15%   86.02%   -0.14%     
==========================================
  Files          16       16              
  Lines        1769     1774       +5     
  Branches      198      198              
==========================================
+ Hits         1524     1526       +2     
- Misses        185      189       +4     
+ Partials       60       59       -1     

Files with missing lines	Coverage Δ
src/testcontainers/core/container.py	84.98% <100.00%> (+0.24%)	⬆️

... and 1 file with indirect coverage changes

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