feat: add delete_generation_output_paths to repo-config.yaml

[jskeet]

This is used as an emergency escape hatch when generation creates files we don't want. The only known usage at the moment is for storage, where files are generated under
internal/generated/snippets/storage/internal and we don't want them.

Fixes googleapis/librarian#2620

[gemini-code-assist]

Summary of Changes

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This pull request provides a crucial "escape hatch" for the code generation process by allowing the explicit deletion of unwanted files or directories after generation. This addresses scenarios where certain outputs are difficult to prevent from being generated but should not be present in the final repository, improving the cleanliness and correctness of generated codebases.

Highlights

• New Configuration Field: Introduced delete_generation_output_paths to repo-config.yaml within the ModuleConfig structure. This field allows specifying a list of file or directory paths to be removed from the generated output.
• Post-Generation Cleanup: Implemented a new deleteOutputPaths function that is invoked at the end of the code generation process. This function iterates through the configured paths and recursively deletes them from the output directory.
• Test Coverage: Added a dedicated test case, TestDeleteOutputPaths, to ensure the correct functionality of the new cleanup mechanism, verifying that specified files and directories are indeed removed.

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[gemini-code-assist]

Code Review

This pull request adds a new configuration delete_generation_output_paths to allow deleting generated files. The implementation has a path traversal vulnerability in deleteOutputPaths which could allow deleting files outside the intended directory. I've suggested a fix for this. Additionally, the new test for this feature is missing a case to check for this vulnerability, so I've suggested an update to the test to include it.