[Drain] Expose drain to dofn processElement and onTimer - add missing implementation in ProcessContext

[stankiewicz]

Some breakage in SpannerIO.readChangeStream at runtime allowed me to find outstanding causedByDrain implementations in ProcessContext that were partially implemented in PR #37825

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Summary of Changes

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This pull request addresses a runtime regression in SpannerIO.readChangeStream by providing a missing implementation for the drain mechanism within the DoFn execution context. This ensures that the harness correctly handles drain signals during element processing.

Highlights

• Implementation of causedByDrain: Added the missing causedByDrain method implementation to the ProcessContext within FnApiDoFnRunner to ensure proper drain handling.

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