[RunInference] Honor model updates in also in the first batch.

[tvalentyn]

1. Honor model updates in first batch.

2. Don't initiate load model calls when there are no updates. In another branch, do an update if the model was invalidated.

3. Drive-by entries to changes.md from earlier PRs.

Fixes: #40665

[github-actions]

📋 Review Summary

This PR resolves a bug (#40665) where RunInference with per-key models (KeyedModelHandler) failed to apply KeyModelPathMapping side-input updates on the first batch when load_model_at_runtime=True. It also avoids redundant update_model calls on every batch when the side input is empty or the per-key model path mapping has not changed.

🔍 General Feedback

• The fix in _RunInferenceDoFn._load_model and process is clean, well-tested, and properly addresses both the first-batch stale model bug and redundant update_model invocations.
• One minor edge case was noted in process when using ModelMetadata side inputs together with timeout-based model invalidation, along with a minor CHANGES.md template placeholder removal.

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💡 Tip: Comment /gemini review on this pull request to trigger a new Gemini code review.

[tvalentyn]

/gemini review

[github-actions]

📋 Review Summary

This PR fixes a bug (#40665) where RunInference with per-key models (KeyedModelHandler) failed to apply KeyModelPathMapping side-input updates on the first batch when load_model_at_runtime=True. It also prevents redundant update_model calls on every batch when side inputs are empty or unchanged, while properly handling model reloading across all branches when a model is marked invalid.

🔍 General Feedback

• The updates in _RunInferenceDoFn._load_model and _RunInferenceDoFn.process are clean, consistent across all side-input branches, and well-covered by the new unit tests.

---

💡 Tip: Comment /gemini review on this pull request to trigger a new Gemini code review.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.02%. Comparing base (76f6e6d) to head (3cc35f6).
⚠️ Report is 240 commits behind head on master.

Additional details and impacted files

@@              Coverage Diff              @@
##             master   #40667       +/-   ##
=============================================
+ Coverage     56.16%   80.02%   +23.85%     
=============================================
  Files          1124      490      -634     
  Lines        178336    82810    -95526     
  Branches       1489        0     -1489     
=============================================
- Hits         100170    66267    -33903     
+ Misses        75645    16543    -59102     
+ Partials       2521        0     -2521     

Flag	Coverage Δ
go	?
java	?
python	80.02% <100.00%> (+0.46%)	⬆️

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[github-actions]

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