Only feed total memory readings to the ModelManager resource estimator

[schizophrenicmaniac]

ModelManager._spawn_new_model was recording peak_during_load - baseline_snap as an observation for {tag: 1}, while release_model records total peak memory for the snapshot of all loaded models. Since ResourceEstimator._solve fits Σ count·cost + bias ≈ reading, the delta rows and total rows contradict each other. With a single model loaded, both kinds even end up in the same history bucket. The delta was also noisy, because the peak window isn't reset before a load.

This removes the spawn-time observation, so the estimator now only sees total memory readings with the full model snapshot from release_model. Profiling unknown models in isolation mode is unchanged, since the initial estimate is still set on release from the isolation baseline.

I added a test that loads two known models at the same time and checks that the estimator history only contains total readings for the combined snapshot. It fails on master, where a 2000 MB delta is recorded for {model_b: 1} while 3000 MB is in use.

Fixes #40636

[github-actions]

📋 Review Summary

This PR cleanly resolves #40636 by removing the spawn-time memory delta observation in ModelManager._spawn_new_model, ensuring ResourceEstimator only receives consistent total memory readings alongside the full active model snapshot during release_model. The change is well-scoped and includes a clear unit test verifying the fix.

🔍 General Feedback

• Removing the delta observation from _spawn_new_model eliminates the contradiction in the NNLS solver (where a bias term is fit against total memory) and prevents noisy readings from unreset peak windows during concurrent model loads.
• Isolation mode profiling for unknown models remains intact via set_initial_estimate on model release.
• The new unit test test_model_manager_observations_use_total_memory accurately exercises the multi-model scenario and verifies the estimator history state.

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[codecov]

Codecov Report

❌ Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.94%. Comparing base (76f6e6d) to head (4180712).
⚠️ Report is 216 commits behind head on master.

Files with missing lines	Patch %	Lines
...s/python/apache_beam/ml/inference/model_manager.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             master   #40646       +/-   ##
=============================================
+ Coverage     56.16%   79.94%   +23.77%     
=============================================
  Files          1124      488      -636     
  Lines        178336    82558    -95778     
  Branches       1489        0     -1489     
=============================================
- Hits         100170    65999    -34171     
+ Misses        75645    16559    -59086     
+ Partials       2521        0     -2521     

Flag	Coverage Δ
go	?
java	?
python	79.94% <0.00%> (+0.38%)	⬆️

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[github-actions]

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