[Deps-Test] Deep-merge injected ExtraChefAttributes with the cluster config's existing ones

[himani2411]

Description of changes

A test feature (e.g. gb200) may
already set DevSettings.Cookbook.ExtraChefAttributes on the cluster config. The
previous 'inject only if absent' guard then skipped our attributes entirely(
ExtraChefAttributes (python-version,nvidia, etc.) via --extra-chef-attributes), so
cluster init used the cookbook default python and failed with
'Errno::ENOENT: .../pyenv/versions/3.14.2/.../python' (the AMI carries the
upgraded python).

Deep-merge instead: parse the existing JSON string, recursively merge our injected attributes under it, and re-serialize. The two dicts share branch keys (cluster, cluster.nvidia) but disjoint children, so a shallow merge would drop one subtree; recursion preserves both. Test-specific attributes win on leaf conflicts (existing passed as the override arg).

Tests

• ONGOING test

References

• Link to impacted open issues.
• Link to related PRs in other packages (i.e. cookbook, node).
• Link to documentation useful to understand the changes.

Checklist

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.92%. Comparing base (bdf724a) to head (8263c8b).
⚠️ Report is 12 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #7459   +/-   ##
========================================
  Coverage    89.91%   89.92%           
========================================
  Files          180      180           
  Lines        16200    16226   +26     
========================================
+ Hits         14567    14592   +25     
- Misses        1633     1634    +1     

Flag	Coverage Δ
unittests	89.92% <ø> (+<0.01%)	⬆️

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