perf: Optimize TTS pool using deadpool (Issue #32)

[NTLx]

This PR implements a proper object pool for TTS sessions using deadpool, addressing Issue #32.

This contribution is part of the OceanBase AI Coding Event.

🚀 Improvements

• Robust Pooling: Replaced the manual channel-based worker implementation with deadpool, a battle-tested object pooling library for Rust.
• Concurrency: Allows dynamic management of TTS sessions instead of a fixed worker loop.
• Simplicity: Reduced boilerplate code by leveraging deadpool's lifecycle management.

🛠 Implementation

• Added deadpool = "0.10" to Cargo.toml.
• Implemented TTSManager struct which implements deadpool::managed::Manager.
• Refactored TTSSessionPool to wrap deadpool::managed::Pool<TTSManager>.
• Updated run_loop to lease sessions from the pool using pool.get().await.

✅ AI Contribution Details

• Analysis: Identified the existing manual worker pool pattern in tts.rs as a candidate for optimization.
• Refactoring: Rewrote the pool implementation to use the standard deadpool pattern (Manager + Recycle).
• Integration: Seamlessly integrated the new pool into the existing async workflow.

[L-jasmine]

When is an idle object destroyed?