[andypalmi]

Closes #9007

Description

1. Rolling back always replaced the instance's env vars with the snapshot's. For a snapshot that came from another instance, such as one a pipeline stored on the target, that copied the other instance's env vars, hidden ones included, with no warning.
2. The rollback endpoint accepts an optional mergeEnvVars boolean and passes it to importProjectSnapshot. It defaults to false, so API callers that omit it behave as before.
3. The rollback dialog has a "Keep the instance's current environment variables" checkbox, ticked by default. Unticked gives the old replace behaviour.
4. Keep uses the existing merge: keys only in the snapshot are added, existing keys keep their values, keys only on the instance are left alone.
5. The rolled-back audit entry records which option was used and the audit log screen shows it.

Testing

1. New unit test in projectSnapshots_spec.js covers mergeEnvVars true and false, with a key only on the instance and a key only in the snapshot, and checks the audit entry.
2. New audit log unit test for the info field.
3. The Cypress rollback spec checks the checkbox is shown and that the request sends mergeEnvVars: true. I haven't run it.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.54%. Comparing base (107cc8c) to head (39b4ba0).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #9009   +/-   ##
=======================================
  Coverage   79.54%   79.54%           
=======================================
  Files         475      475           
  Lines       26076    26076           
  Branches     6950     6950           
=======================================
  Hits        20741    20741           
  Misses       5335     5335           

Flag	Coverage Δ
backend	79.54% <100.00%> (ø)

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[Steve-Mcl]

@knolleary - I am happy to do review and testing on this if it has your blessing.

[knolleary]

@Steve-Mcl I've added one suggested change to the dialog text based purely on something that caught my eye. I haven't done a deep dive on the implementation - happy for you to handle that.