contrib.piecewise: Add special case for single-segment "piecewise" linear functions

[emma58]

Fixes # .

Summary/Motivation:

@michaelbynum and others have observed that we don't have any way to handle the special case of piecewise-linear in which the model is just plain linear (one segment). This catches that case before all of the GDP/MIP transformations so that we don't create unnecessary Booleans/binaries in this case.

Changes proposed in this PR:

• Adds handling in the transformation base class to return linear expressions for single-segment cases rather than creating disjunctive or other logical structures. This can still be bypassed when it's not relevant, and the epigraph/hypograph transformation (as written in contrib.piecewise: Adding option for convexity/concavity and an epigraph/hypograph transformation #3989) already will.
• Tests single-segment functions for all transformations.

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Notes for reviewers (optional): Note that this is completely independent of #3989: The epigraph/hypograph should not have this same handling because then it does something unexpected, which is to not return an epigraph/hypograph. So the fact that it will still redefine _transform_pw_linear_expr is good.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.14%. Comparing base (9e9902c) to head (ac7a115).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3990   +/-   ##
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  Coverage   90.13%   90.14%           
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  Files         917      917           
  Lines      109091   109101   +10     
=======================================
+ Hits        98333    98344   +11     
+ Misses      10758    10757    -1     

Flag	Coverage Δ
builders	29.09% <50.00%> (+<0.01%)	⬆️
default	86.13% <100.00%> (?)
expensive	35.07% <50.00%> (?)
linux	87.66% <100.00%> (-1.98%)	⬇️
linux_other	87.66% <100.00%> (+<0.01%)	⬆️
oldsolvers	28.03% <50.00%> (-0.01%)	⬇️
osx	83.11% <100.00%> (+<0.01%)	⬆️
win	85.42% <100.00%> (+<0.01%)	⬆️

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