Fix live fleet map settings render-time mutation

[roncodes]

Summary

• keep the map settings in-flight request promise as internal, non-rendered state
• prevent Ember from dirtying the tracked loadPromise tag after it is read during live-fleet widget construction
• resolve the runtime assertion that prevented the live fleet widget from rendering

Verification

Lint and tests were intentionally not run while the reported local runtime errors are being cleared. The change is isolated to addon/services/map-settings.js.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (71e2e8a) to head (116d8aa).
⚠️ Report is 13 commits behind head on release/v0.6.62.

Additional details and impacted files

@@                 Coverage Diff                  @@
##             release/v0.6.62      #314    +/-   ##
====================================================
  Coverage             100.00%   100.00%            
- Complexity              9899     10003   +104     
====================================================
  Files                    526       531     +5     
  Lines                  38163     38585   +422     
====================================================
+ Hits                   38163     38585   +422     

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

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