Fix for SQS transport deadlock

[rlaunch]

This PR addresses a deadlock in the SQS fanout module which caused fanout queue _queue_bind calls to hang indefinitely. These issues were missed by existing unit tests as they were run in isolation.

Parts of the SNS module have been refactored to improve maintainability, and locks are held lower down the stack when initialise_exchange is called. As well as this, improvements have been made to SNS subscription error handling for consistency.

Integration tests have been added for the SNS module to catch issues like this in future.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.23%. Comparing base (3633f91) to head (b9d6822).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2479      +/-   ##
==========================================
- Coverage   82.24%   82.23%   -0.02%     
==========================================
  Files          79       79              
  Lines       10086    10080       -6     
  Branches     1153     1150       -3     
==========================================
- Hits         8295     8289       -6     
  Misses       1589     1589              
  Partials      202      202              

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[auvipy]

is it possible to add extra integration tests for this as well?

[Copilot]

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[rlaunch]

is it possible to add extra integration tests for this as well?

Which module or method did you want more integration tests for?

Best,
Ryan