feat(nf-google): add configurable CLI staging transports and safer default unstaging for Google Batch

[tomiles]

This PR addresses #5653 by adding optional Google Batch stage-in/stage-out copy transports (posix, gcloud, gsutil) and aligning transfer behavior with other cloud executors while preserving backward-compatible defaults.

This design allows granular optimization of staging behavior at the process level (via stageInMode / stageOutMode), rather than forcing a single workflow-wide override. In practice, only processes that use copy for stage-in or stage-out (stage-out defaults to copy in Google Batch when unset) and have the CLI tools in their runtime environment will use the new transport path, while other modes keep existing behavior.

What changed

• Added google.batch.stageInCopyTransport and google.batch.stageOutCopyTransport to select override for transfer transport in copy mode.
• Added Google Batch transfer options:
  • google.batch.gcloudCli
  • google.batch.gsutilCli
  • google.batch.maxParallelTransfers
  • google.batch.maxTransferAttempts
  • google.batch.delayBetweenAttempts
• Added/updated Google Batch bash staging helpers with CLI tool first behavior and POSIX fallback.
• Folded in the Default stageOutMode to 'copy' for Google Batch executor #6917 behavior: default Google Batch unstaging to copy when stageOutMode is unset to avoid move failures in gcsfuse-mounted work-dir workflows (e.g. overlapping outputs/symlinked paths).

Docs

• Updated docs/google.md with the new transport options and default unstaging rationale.
• Updated docs/reference/config.md with all new google.batch.* config entries and defaults.

Acknowledgements

Thanks to @dmvanbeek and @rhassaine for input and hackathon discussions that helped shape this approach.

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