Resolve error identifying vars in indexed SOS

[jsiirola]

Fixes #2827

Summary/Motivation:

The NLv2 writer incorrectly handled iterating over SOSConstraint components when identifying model variables. This PR fixes the issue and adds a test exercising the correct behavior.

Changes proposed in this PR:

• Resolve error iterating over SOSConstraint component data.
• Add test

Legal Acknowledgement

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1. I agree my contributions are submitted under the BSD license.
2. I represent I am authorized to make the contributions and grant the license. If my employer has rights to intellectual property that includes these contributions, I represent that I have received permission to make contributions and grant the required license on behalf of that employer.

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (4743218) 87.45% compared to head (2d1805d) 87.46%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2852   +/-   ##
=======================================
  Coverage   87.45%   87.46%           
=======================================
  Files         771      771           
  Lines       89582    89582           
=======================================
+ Hits        78345    78349    +4     
+ Misses      11237    11233    -4     

Flag	Coverage Δ
linux	84.52% <100.00%> (+<0.01%)	⬆️
osx	74.59% <100.00%> (+0.02%)	⬆️
other	84.69% <100.00%> (ø)
win	82.09% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
pyomo/repn/plugins/nl_writer.py	94.69% <100.00%> (ø)

... and 1 file with indirect coverage changes

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