🧹 [refactor] Extract string operation callbacks into functions

[Tcode-Motion]

🎯 What: Extracted inline string operation callbacks into top-level private functions.
💡 Why: To de-nest the string manipulation exports and improve code readability.
✅ Verification: Ran cargo test --workspace --all-targets and verified no new failures were introduced. Note that pre-existing workspace errors related to bincode, rand, and ureq remain unchanged.
✨ Result: Cleaner and more maintainable standard library registration for strings.

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PR created automatically by Jules for task 17457631423011722919 started by @Tcode-Motion

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