🧹 [De-nest string manipulation exports]

[Tcode-Motion]

🎯 What: Extracted nested string manipulation closures into top-level private functions in stdlib/src/strings.rs.
💡 Why: To improve code readability and maintainability by de-nesting the logic.
✅ Verification: Verified changes with git diff and ran tests using cargo test --workspace --all-targets to ensure no new failures were introduced.
✨ Result: Improved readability and maintainability of the stdlib/src/strings.rs module.

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PR created automatically by Jules for task 7968661651229804843 started by @Tcode-Motion

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