Cast channel_data to float at construction time

[isofer]

Summary

• Cast _channel data to float in MMM._generate_and_preprocess_model_data() immediately after building the xarray dataset, so all downstream consumers (scalers, pm.Data, budget optimizer, incrementality, sensitivity analysis) always see float data.
• Adds a test verifying that integer channel input is cast to float at construction time.

Closes #2340

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📚 Documentation preview 📚: https://pymc-marketing--2341.org.readthedocs.build/en/2341/

[cursor]

PR Summary

Low Risk
Low risk: a small, targeted dtype normalization that affects only channel input tensors; main risk is minor behavior change for integer channel inputs, now always treated as float.

Overview
Casts the merged xarray _channel variable to float during MMM._generate_and_preprocess_model_data(), ensuring downstream scaling and pm.Data('channel_data') consistently operate on floating-point channel inputs.

Adds a regression test confirming that integer channel columns are converted to float in both mmm.xarray_dataset['_channel'] and the constructed PyTensor channel_data node.

^{Written by Cursor Bugbot for commit 7b09473. This will update automatically on new commits. Configure here.}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.72%. Comparing base (fbc2831) to head (7b09473).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2341      +/-   ##
==========================================
- Coverage   93.18%   89.72%   -3.47%     
==========================================
  Files          79       79              
  Lines       12472    12473       +1     
==========================================
- Hits        11622    11191     -431     
- Misses        850     1282     +432     

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[ricardoV94]

can't you just do astype(float) regardless?

[isofer]

done