Review security of postStart shell interpolation

Issue: Review security of user-provided postStart shel... (#547719 - closed)

What does this MR do and why?

Adds proper shell escaping and security review comments. The main improvements are:

1. Added shell escaping: When the system builds shell scripts that run commands, it now properly escapes file paths to prevent malicious code injection. This is like putting quotes around file names to ensure they're treated as literal text rather than executable commands.

2. Added security review comments: The developers added detailed comments explaining that they've reviewed each place where user input or system paths are used in shell commands, documenting what data is safe and why.

3. Refactored script generation: Instead of using Ruby's heredoc syntax (multi-line strings), the code now builds scripts line by line and uses proper escaping functions. This makes the code safer and more explicit about how shell commands are constructed.

4. Improved code structure: The script generation logic was reorganized to be clearer and more maintainable, separating the content generation from the final script assembly.

These changes protect against potential security vulnerabilities where malicious file names or paths could be interpreted as shell commands, while making the code's security measures more transparent and maintainable.

References

Screenshots or screen recordings

This MR does not contain any UI changes

Smoke test result:

How to set up and validate locally

Run in terminal: bundle exec scripts/remote_development/run-smoke-test-suite.sh

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