🧹 [Code Health] Reduce deep nesting in register_ai

[Tcode-Motion]

🎯 What: Extracted the deeply nested blocks for "openai", "gemini", and "local" inside register_ai (in stdlib/src/ai.rs) into separate module-level helper functions (generate_openai, generate_gemini, generate_local).
💡 Why: Reduces nesting, making the register_ai match statement clean, concise, and easier to maintain or extend in the future.
✅ Verification: Ran cargo check --workspace and cargo test --workspace to confirm the compilation succeeds and no existing tests were broken by the change.
✨ Result: Enhanced the readability and maintainability of the codebase without changing external behavior or logic.

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PR created automatically by Jules for task 7944786552527752162 started by @Tcode-Motion

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