[damccorm]

Follow-up to #40906. We were duplicating a ton of logic there and in other PRs, so this is trying to simplify things.

Refactors the single-base ModelHandler wrapper classes (MaybeKeyedModelHandler, _PrebatchedModelHandler, _PreProcessingModelHandler, and _PostProcessingModelHandler) to inherit from a shared _DelegatingModelHandler base class instead of duplicating ~15 pass-through methods on each wrapper class.

Also forwards set_environment_vars() to the underlying ModelHandler in _DelegatingModelHandler (and single-model KeyedModelHandler) so custom set_environment_vars() overrides on wrapped handlers are preserved alongside _env_vars.

[codecov]

Codecov Report

❌ Patch coverage is 98.07692% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 57.46%. Comparing base (d3078c1) to head (052c7fc).
⚠️ Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
sdks/python/apache_beam/ml/inference/base.py	98.07%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #41051      +/-   ##
============================================
- Coverage     57.48%   57.46%   -0.02%     
  Complexity     2492     2492              
============================================
  Files          1204     1204              
  Lines        171265   171181      -84     
  Branches       2103     2103              
============================================
- Hits          98456    98375      -81     
+ Misses        70157    70154       -3     
  Partials       2652     2652              

Flag	Coverage Δ
python	80.20% <98.07%> (-0.02%)	⬇️

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[github-actions]

📋 Review Summary

This PR cleanly deduplicates the single-base ModelHandler wrapper classes (MaybeKeyedModelHandler, _PrebatchedModelHandler, _PreProcessingModelHandler, and _PostProcessingModelHandler) by introducing a shared _DelegatingModelHandler base class. It also properly delegates set_environment_vars() across wrappers and single-model KeyedModelHandler instances, with solid unit test coverage.

🔍 General Feedback

• The _DelegatingModelHandler abstraction significantly reduces boilerplate (~140 net lines removed) and ensures future ModelHandler hooks only need to be forwarded in one place.
• Forwarding set_environment_vars() and preserving _model_identifier in _DelegatingModelHandler.__init__ improves consistency across wrapper chains.
• Updating _is_multi_model_keyed_handler to unwrap any _DelegatingModelHandler makes the check more general and covers MaybeKeyedModelHandler as well.

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[github-actions]

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