Allow edit operation to reuse proxy port of the old proxy

[dmjb]

Prior to this PR, the edit API endpoint would not allow the proxy port to be reused. This was because the run config validation happened before stopping the old instance of the workload, and thus the port appeared to be still in use.

This PR adds some tweaks to the validation logic to allow port reuse for the edit case. It also tweaks the edit endpoint to reuse the previous port of the workload if a proxy port is not specified - this feels like a more natural behaviour than picking a new random port. This implementation also allows a new port to be selected if the caller desires.

There is a potential race condition in this implementation: if a new workload is created between the termination of the previous proxy process and the creation of the new one, it could potentially "steal" the port and cause the edit to fail after validation has completed. I think this is an acceptable risk for now - fixing it will require us to build some kind of locking mechanism for ports.

Fixes: #3182

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 57.18%. Comparing base (063ed3f) to head (163d8f9).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3200      +/-   ##
==========================================
+ Coverage   57.11%   57.18%   +0.07%     
==========================================
  Files         346      346              
  Lines       34314    34327      +13     
==========================================
+ Hits        19598    19631      +33     
+ Misses      13091    13061      -30     
- Partials     1625     1635      +10     

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