chore: update mrd doc with special cases and optimize cloudbuild.yaml

[chandra-siri]

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Fixes #<issue_number_goes_here> 🦕

[gemini-code-assist]

Summary of Changes

Hello @chandra-siri, 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 focuses on two distinct areas: optimizing the Cloud Build configuration for system tests and enhancing documentation for an experimental asynchronous multi-range downloader. The Cloud Build changes streamline SSH key management, while the documentation updates provide clearer guidance on specific parameter usage, improving overall clarity and efficiency.

Highlights

• Cloud Build Optimization: Streamlined SSH key handling in cloudbuild/zb-system-tests-cloudbuild.yaml by removing an intermediate file copy and directly referencing the generated public key for cleanup, simplifying the build process.
• Documentation Enhancement: Updated the documentation for the async_multi_range_downloader.py module, specifically clarifying the special meaning of (0, 0) for start_byte and bytes_to_read parameters in the download_ranges function, indicating a full file download.

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[gemini-code-assist]

Code Review

This pull request aims to optimize a Cloud Build configuration and update documentation. However, the Cloud Build change introduces a critical issue by specifying an incorrect path for an SSH key file, which will cause the key cleanup step to fail silently. Additionally, the documentation update for async_multi_range_downloader.py includes an unclear and likely erroneous entry that needs correction for clarity.

[gemini-code-assist]

The path to the public key file appears to be incorrect. The key is generated at /workspace/.ssh/google_compute_engine.pub, but this step attempts to access it from /.ssh/google_compute_engine.pub. Since /workspace is persisted across build steps, the correct absolute path should be used. The || true will prevent the build from failing, but the SSH key will not be removed from the OS Login profile, leading to an accumulation of keys over time, which this step is intended to prevent.

          --key-file=/workspace/.ssh/google_compute_engine.pub || true

[gemini-code-assist]

The documentation for special meanings of start_byte and bytes_to_read seems incomplete or contains a typo. The line with just () is unclear and could be confusing to users. Please clarify what this represents or remove it if it's an error.