bindings/rust: Add opt-in Database connection pooling

[ongyimeng]

Description

Added opt-in connection pooling to Rust bindings Database:

• New Database::connect_pooled() API
• New builder option max_idle_connections(...) (default 16)
• Added internal ConnectionPool and PooledConnection wrapper (RAII return-to-pool on drop)
• Added safety checks before recycling connections in connection.rs (can_recycle_into_pool, reset_for_reuse)
• Added/expanded unit + integration tests for pooled behavior (reuse, disabled pool, capacity, concurrency, into_inner, mutable-method forwarding)

Motivation and context

Apps currently need to build their own pooling around Database. This change provides built-in pooling while keeping existing behavior unchanged for connect() users (pooling is opt-in via connect_pooled())

Issue #5721

Description of AI Usage

• locating relevant Rust binding files and APIs
• drafting the pooling implementation and test cases
• running and iterating on validation commands (cargo fmt, cargo test -p turso, cargo clippy ... --deny=warnings)

The final code, tests, and behavior were reviewed and validated by the committer.

[turso-bot]

Please review @LeMikaelF

[ongyimeng]

@PThorpe92 not sure who else to ask, but would you mind reviewing this when you get a chance?