fix: 修复代码审查发现的问题

[zx06]

1. 修复 MySQL driver 资源泄漏：使用 sync.Once 替代每次 Open 都注册 dialer
2. 提取公共连接逻辑到 internal/app：新增 ResolveConnection 和 ResolveSSH 函数
3. 重构 cmd/xsql 使用公共连接助手，消除代码重复
4. 更新测试使用新的 app.ResolveSSH 函数

[codecov]

Codecov Report

❌ Patch coverage is 47.16981% with 56 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.85%. Comparing base (6bd2c00) to head (6c44548).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
internal/app/conn.go	40.00%	47 Missing and 1 partial ⚠️
internal/db/mysql/driver.go	63.63%	3 Missing and 1 partial ⚠️
cmd/xsql/query.go	71.42%	2 Missing ⚠️
cmd/xsql/schema.go	71.42%	2 Missing ⚠️

❌ Your patch check has failed because the patch coverage (47.16%) is below the target coverage (80.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #30      +/-   ##
==========================================
+ Coverage   80.76%   80.85%   +0.08%     
==========================================
  Files          36       37       +1     
  Lines        2392     2403      +11     
==========================================
+ Hits         1932     1943      +11     
- Misses        357      361       +4     
+ Partials      103       99       -4     

Flag	Coverage Δ
e2e	44.76% <0.00%> (-0.22%)	⬇️
integration	44.76% <0.00%> (-0.22%)	⬇️
unittests	66.45% <47.16%> (+0.19%)	⬆️

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[sonarqubecloud]

Quality Gate failed

Failed conditions
3 Security Hotspots

See analysis details on SonarQube Cloud