Gams/Baron: ensure negative numbers are parenthesized

[jsiirola]

Fixes #2819 .

Summary/Motivation:

This resolves an issue where negative constants were not being parenthesized, which lead to incorrect expressions in certain contexts (like exponentiation)

Changes proposed in this PR:

• ensure negative constants are always parenthesized
• add test for issue BARON Writer Outputs Inconsistent Constraint Expressions for Model with Fixed Var Objects  #2819

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[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.01 ⚠️

Comparison is base (47deef1) 87.18% compared to head (e20b685) 87.17%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2833      +/-   ##
==========================================
- Coverage   87.18%   87.17%   -0.01%     
==========================================
  Files         765      765              
  Lines       88471    88457      -14     
==========================================
- Hits        77134    77113      -21     
- Misses      11337    11344       +7     

Flag	Coverage Δ
linux	84.25% <100.00%> (+<0.01%)	⬆️
osx	73.90% <100.00%> (ø)
other	84.42% <100.00%> (ø)
win	81.79% <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/gams_writer.py	86.00% <ø> (ø)
pyomo/repn/plugins/baron_writer.py	82.11% <100.00%> (ø)

... and 14 files with indirect coverage changes

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