feat(driver): implement HTTP connection pool, db.connect/disconnect and CI (#9)

[ZhuchkaTriplesix]

Summary of Changes

• Implemented robust ClickHouseClient in src/driver/client.rs with automatic URL parsing, Safe Mode (readonly=1) enforcement, and zero-trust authentication header injection from ConnectionSecretsPool.
• Implemented global thread-safe session registry ConnectionPool in src/driver/pool.rs.
• Implemented db.connect and db.disconnect JSON-RPC handlers in src/rpc/handlers/connection.rs with live health check (SELECT version()) and mock testing support (mock://).
• Added GitHub Actions CI Workflow in .github/workflows/rust.yml enforcing formatting (cargo fmt), linting (cargo clippy -- -D warnings), and unit tests (cargo test).
• Added 4 new unit tests (19 total tests passing cleanly).

Closes #9