Fix differentiation of Expressions containing native_numeric_types

[jsiirola]

Fixes #2986

Summary/Motivation:

Recently, Pyomo changed the Expression object API so that .expr would always return a class derived from NumericValue, but not necessarily the same object each time it was called. As a result, walkers that key off the expression child need to use .arg(0) and not .expr. This PR resolves that bug in the Pyomo reverse_ad walker.

Changes proposed in this PR:

• Use .arg(0) and not .expr when accessing the child of an Expression
• Add a test exercising the behavior reported in Pyomo AD fails with constant Expressions #2986

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[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (4267cbd) 87.92% compared to head (4c13589) 86.72%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3017      +/-   ##
==========================================
- Coverage   87.92%   86.72%   -1.20%     
==========================================
  Files         769      769              
  Lines       89538    89538              
==========================================
- Hits        78727    77654    -1073     
- Misses      10811    11884    +1073     

Flag	Coverage Δ
linux	83.98% <100.00%> (-1.20%)	⬇️
osx	74.97% <100.00%> (ø)
other	84.16% <100.00%> (-1.20%)	⬇️
win	82.40% <100.00%> (ø)

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Files	Coverage Δ
pyomo/core/expr/calculus/diff_with_pyomo.py	98.97% <100.00%> (ø)

... and 13 files with indirect coverage changes

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