fix: preserve 2D outputs for (L,1) inputs

[MilesCranmerBot]

Fixes #571.

• _SingleSymPyModule.forward now:
  • accepts 1D inputs by unsqueezing to (L,1) (helps module composition)
  • preserves (L,1) output shape when the effective input is a single feature column
• Adds regression tests covering shape preservation + simple module composition.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.27%. Comparing base (c435527) to head (a342033).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1137      +/-   ##
==========================================
+ Coverage   94.25%   94.27%   +0.02%     
==========================================
  Files          21       21              
  Lines        1687     1693       +6     
==========================================
+ Hits         1590     1596       +6     
  Misses         97       97              

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