[YAML] Fix flaky ErrorHandlingTest: don't pickle Scope into user DoFn closures

[shunping]

Summary

Fixes flaky ErrorHandlingTest.test_error_handling_outputs and ErrorHandlingTest.test_strip_error_metadata in yaml_transform_test.py, which intermittently fail with:

    RuntimeError: Unable to pickle fn CallableWrapperDoFn(...):
    cannot pickle 'google._upb._message.Descriptor' object

Root cause

• Scope.create_ptransform monkey-patches annotations and expand onto the user's transform instance. The patched expand (recording_expand) closes over the YAML Scope, which references every provider, including the process-global standard_providers() (cached via functools.cache).
• Any DoFn that captures its enclosing transform (e.g. SizeLimiter's raise_on_big uses self._limit) therefore pickles the whole Scope and all providers. Providers hold lazily-populated, process-wide state (e.g. ExternalProvider._service / _schema_transforms), so whether the pickle succeeds depends on which tests ran earlier on the same xdist worker. Hence the flakiness.
• Locally, the pickled SizeLimiter closure is ~90 KB and contains the Scope, 28 ExternalJavaProviders, 20 RenamingProviders, etc., all to carry a single int.

Fix

Add Scope.__reduce__ returning a string stub, mirroring the existing Pipeline.__reduce__.
The Scope is only used during pipeline construction:

• recording_expand only runs during runner.apply_PTransform; workers never call a composite's expand.
• Annotations are read once when the transform is applied and encoded into the pipeline proto.
• Transforms restored via PICKLED_TRANSFORM are never re-expanded.

This keeps the existing monkey-patching unchanged, so there's no change to transform labels (update compatibility) or to type-hint behavior. After the fix, the pickled closure drops from ~90 KB to ~3.9 KB.

[shunping]

r: @derrickaw as the author of the previous attempt (#39845).

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.55%. Comparing base (c275c97) to head (3b9acca).
⚠️ Report is 94 commits behind head on master.

Additional details and impacted files

@@              Coverage Diff              @@
##             master   #40467       +/-   ##
=============================================
+ Coverage     56.20%   79.55%   +23.34%     
=============================================
  Files          1123      487      -636     
  Lines        178213    82238    -95975     
  Branches       1489        0     -1489     
=============================================
- Hits         100168    65422    -34746     
+ Misses        75524    16816    -58708     
+ Partials       2521        0     -2521     

Flag	Coverage Δ
go	?
java	?
python	79.55% <100.00%> (-0.13%)	⬇️

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