Reduce fetches to VMSS

[Porges]

Remove another GET from ListInstanceIds, and only invoke ListInstanceIds if we have to (i.e. one of the nodes is missing InstanceId).

[codecov-commenter]

Codecov Report

Merging #2577 (58b77f3) into main (4b305d0) will decrease coverage by 5.09%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main    #2577      +/-   ##
==========================================
- Coverage   29.89%   24.79%   -5.10%     
==========================================
  Files         289      120     -169     
  Lines       35767    12486   -23281     
==========================================
- Hits        10692     3096    -7596     
+ Misses      25075     9390   -15685     

Impacted Files	Coverage Δ
...ce/ApiService/TestHooks/NodeOperationsTestHooks.cs	0.00% <0.00%> (ø)
...ApiService/ApiService/onefuzzlib/NodeOperations.cs	23.31% <0.00%> (-0.35%)	⬇️
...ervice/ApiService/onefuzzlib/ScalesetOperations.cs	2.67% <0.00%> (+<0.01%)	⬆️
...ApiService/ApiService/onefuzzlib/VmssOperations.cs	0.00% <0.00%> (ø)
src/agent/coverage/src/cobertura.rs
...rc/agent/onefuzz-task/src/tasks/fuzz/supervisor.rs
...c/agent/onefuzz-task/src/local/generic_analysis.rs
src/agent/onefuzz-agent/src/reboot/double.rs
src/agent/srcview/src/pdbcache.rs
... and 164 more

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