{vm-repair} Add code documentation and fix Unbound exceptions

[Sandido]

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This PR is an example of using AI to add documentation to code and Github Copilot to find bugs to fix.
This PR adds code documentation to the big custom.py file.
This also fixes an Unbound exception bug in the existing_rg and copy_disk_id variables.
Fixed a duplicate test name error.

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• Have you run azdev style <YOUR_EXT> locally? (pip install azdev required)
• Have you run python scripts/ci/test_index.py -q locally? (pip install wheel==0.30.0 required)
• My extension version conforms to the Extension version schema

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• My extension description/summary conforms to the Extension Summary Guidelines.

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You only need to update the version information in file setup.py and historical information in file HISTORY.rst in your PR but do not modify src/index.json.

[azure-client-tools-bot-prd]

️✔️Azure CLI Extensions Breaking Change Test

️✔️Non Breaking Changes

[azure-client-tools-bot-prd]

Hi @Sandido,
Please write the description of changes which can be perceived by customers into HISTORY.rst.
If you want to release a new extension version, please update the version in setup.py as well.

[yonzhan]

Thank you for your contribution! We will review the pull request and get back to you soon.

[github-actions]

• For more info about extension versioning, please refer to Extension version schema

[Sandido]

@yanzhudd , please re-review and merge if possible.

[yanzhudd]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 2 pipeline(s).

[azclibot]

[Release] Update index.json for extension [ vm-repair ] : https://dev.azure.com/msazure/One/_build/results?buildId=109101394&view=results