Add idx_exclude to MarginalizingTimingModel

[vhaasteren]

Summary

• Adds an idx_exclude parameter to MarginalizingTimingModel, mirroring the existing API on TimingModel.
• Excluded columns are dropped before basis construction (including the SVD path), so the marginalized timing-model matrix only spans the remaining linear parameters.
• Adds unit tests for default behavior, column exclusion, and SVD + exclusion, plus a likelihood smoke test.

This is a small API parity change. It lets callers marginalize over a subset of timing-model columns while handling the excluded ones elsewhere — for example, when a nonlinear delay term samples some parameters and the remaining linear timing parameters are marginalized analytically.

Test plan

• pytest tests/test_gp_signals.py::TestGPSignals::test_marginalizing_timing_model_idx_exclude
• pytest tests/test_likelihood.py::TestLikelihood::test_like_marginalizing_tm_idx_exclude
• Full Enterprise test suite (if CI is available)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.71%. Comparing base (ebadc2f) to head (5b3d80c).
⚠️ Report is 99 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #451      +/-   ##
==========================================
+ Coverage   72.59%   72.71%   +0.11%     
==========================================
  Files          13       13              
  Lines        3478     3478              
==========================================
+ Hits         2525     2529       +4     
+ Misses        953      949       -4     

Files with missing lines	Coverage Δ
enterprise/signals/gp_signals.py	69.87% <100.00%> (ø)

... and 8 files with indirect coverage changes

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