Fix trigger example write append

[RushabhRatnaparkhi]

Fixes #18767

The TriggerExample runs multiple pipelines that write to the same BigQuery table.
The default WRITE_EMPTY disposition causes the example to fail on Dataflow after
the first pipeline creates the table.

This change explicitly sets WRITE_APPEND so all trigger pipelines can write
successfully to the same table.

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[gemini-code-assist]

Summary of Changes

Hello @RushabhRatnaparkhi, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request resolves an issue in the TriggerExample where multiple pipelines attempting to write to the same BigQuery table would encounter failures after the initial table creation. By configuring the BigQuery sink to use WRITE_APPEND disposition, all subsequent writes are now correctly handled, ensuring the example's robust execution, particularly within Dataflow environments.

Highlights

• BigQuery Write Disposition: Explicitly set BigQueryIO.Write.WriteDisposition.WRITE_APPEND for BigQuery writes in the TriggerExample to resolve failures when multiple pipelines attempt to write to the same table.

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[Copilot]

Pull request overview

This PR fixes a BigQuery write failure in the TriggerExample cookbook example when running on Dataflow. The example runs four separate pipelines with different trigger configurations that all write to the same BigQuery table. Previously, the default WRITE_EMPTY disposition caused failures after the first pipeline created the table.

Changes:

• Added explicit WRITE_APPEND write disposition to the BigQuery write operation in TriggerExample

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[Copilot]

The indentation for the BigQueryIO builder chain is inconsistent with other examples in the codebase. Most other examples (like AutoComplete.java:513-520, CombinePerKeyExamples.java:197-201, WriteToBigQuery.java:129-134) use 4-space indentation for chained methods, but this code uses a larger indentation (10 spaces). Consider adjusting the indentation to match the established pattern in the codebase, aligning the chained methods at 4 spaces relative to the method call context.

[github-actions]

Assigning reviewers:

R: @chamikaramj for label java.

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