[BEAM-11106] Validate that DoFn returns Process continuation when Truncating

[riteshghorse]

We need to restrict the Truncate method to only DoFns with ProcessContinuations

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[riteshghorse]

R: @lostluck

[codecov]

Codecov Report

Merging #17770 (c84d977) into master (9a67e30) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master   #17770      +/-   ##
==========================================
- Coverage   74.00%   74.00%   -0.01%     
==========================================
  Files         695      695              
  Lines       91841    91850       +9     
==========================================
+ Hits        67971    67974       +3     
- Misses      22624    22630       +6     
  Partials     1246     1246              

Flag	Coverage Δ
go	50.47% <100.00%> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
sdks/go/pkg/beam/core/graph/fn.go	77.03% <100.00%> (+0.20%)	⬆️
sdks/python/apache_beam/internal/gcp/auth.py	73.33% <0.00%> (-5.34%)	⬇️
.../python/apache_beam/testing/test_stream_service.py	88.09% <0.00%> (-4.77%)	⬇️
...n/apache_beam/ml/gcp/recommendations_ai_test_it.py	73.46% <0.00%> (-2.05%)	⬇️
...python/apache_beam/runners/worker/worker_status.py	78.26% <0.00%> (-1.45%)	⬇️
...che_beam/runners/interactive/interactive_runner.py	89.20% <0.00%> (-1.44%)	⬇️
...on/apache_beam/examples/dataframe/flight_delays.py	27.90% <0.00%> (-0.67%)	⬇️
...apache_beam/runners/portability/portable_runner.py	77.31% <0.00%> (+1.71%)	⬆️
sdks/python/apache_beam/utils/interactive_utils.py	97.56% <0.00%> (+2.43%)	⬆️

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[lostluck]

LGTM. Thanks!