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 93.93%. Comparing base (2ac7e71) to head (275423d).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1137      +/-   ##
==========================================
+ Coverage   93.91%   93.93%   +0.02%     
==========================================
  Files          21       21              
  Lines        1643     1649       +6     
==========================================
+ Hits         1543     1549       +6     
  Misses        100      100              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.