feat: Add unified Execute interface design in client_design.md

- Add comprehensive Execute interface with automatic statement routing
- Support parameterized queries with type safety
- Add OpenTelemetry integration design with interceptor pattern
- Fix mermaid syntax errors in all diagrams (added missing classDiagram declarations)
- Fix both English and Chinese versions

This provides a cleaner unified API for SQL-like operations while
maintaining backward compatibility with existing methods.

Signed-off-by: mikkeyf <1647228132@qq.com>

Author: Mikkey-f