[examples] Atomically publish subprocess executables

[bvolpato]

ExampleEchoPipelineTest intermittently fails on Linux while launching a freshly copied shell executable:

java.io.IOException: Cannot run program ".../test-EchoAgain....sh":
Exec failed, error: 26 (Text file busy)

This occurred twice in scheduled Java precommit runs, including https://github.com/apache/beam/actions/runs/30802709511.

FileUtils.copyFileFromGCSToWorker currently copies directly to the executable's final path. Its in-process lock does not protect separate classloaders or worker processes sharing that path, so another process can try to execute the destination while it is still open for writing.

This change stages each executable in a temporary sibling file, makes the staged file executable, then atomically replaces the destination. Readers therefore see either the previous complete executable or the new complete executable. Launch retries are intentionally avoided so unrelated ProcessBuilder failures remain visible.

Reproduction

A focused Linux regression test holds the previous destination inode open for writing while publishing its replacement, then launches the destination path. Before this change it fails deterministically with error=26, Text file busy; after this change it succeeds.

Testing

• FileUtilsTest and ExampleEchoPipelineTest: 10 consecutive passes
• ./gradlew :examples:java:spotlessCheck :examples:java:checkstyleMain :examples:java:checkstyleTest --no-build-cache
• Full :examples:java:test attempted: 69 tests passed; unrelated KafkaToPubsubE2ETest class setup could not start because Docker is unavailable locally

[github-actions]

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R: @chamikaramj for label java.

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