pyrenew_builder delegates validation of observational data to observation classes

[cdc-mitzimorris]

Issue #678: Delegate observation data validation to observation processes

Changes made:

1. pyrenew/observation/base.py - Added abstract validate_data(n_total, n_subpops, **obs_data) method plus three reusable validation helpers: _validate_times(),
   _validate_subpop_indices(), _validate_obs_times_length()
2. pyrenew/observation/count_observations.py - Implemented validate_data() on Counts (validates dense obs length) and CountsBySubpop (validates times, subpop_indices, obs/times
   length match)
3. pyrenew/observation/measurements.py - Implemented validate_data() on Measurements (validates times, subpop_indices, sensor_indices, obs/times length match)
4. pyrenew/model/multisignal_model.py - Replaced 46 lines of hardcoded validation with 4-line delegation loop
5. test/test_pyrenew_builder.py - Added validation_builder fixture with both Counts and CountsBySubpop observations; updated tests to target the correct observation type; added
   dense obs length test; made match patterns less brittle
6. test/test_observation_counts.py - Added validate_data stub to test's IncompleteCountProcess

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.73%. Comparing base (e7df19c) to head (db7c53d).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #698      +/-   ##
==========================================
+ Coverage   97.43%   97.73%   +0.29%     
==========================================
  Files          51       51              
  Lines        1523     1542      +19     
==========================================
+ Hits         1484     1507      +23     
+ Misses         39       35       -4     

Flag	Coverage Δ
unittests	97.73% <100.00%> (+0.29%)	⬆️

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[github-actions]

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[dylanhmorris]

LGTM! A few minor suggestions.

[cdc-mitzimorris]

@dylanhmorris - fixed inconsistent/incomplete data validation - moved logic into base class. thanks for being picky.

[dylanhmorris]

Thanks @cdc-mitzimorris! I think you can fix pre-commit issues and merge