Use type system to prevent request validation bypasses

## Summary
We should refactor the proxy request handling to use the type system to ensure all requests passed to upstream servers have been validated by the RuleEngine, making it impossible at compile time to accidentally bypass security checks.

## Background
While fixing the Host header bypass vulnerability (#57), we added runtime validation to ensure the Host header matches the URI. However, this and similar security checks could be better enforced through the type system.

## Proposed Solution
Introduce type-safe request handling that guarantees validation:

```rust
// Example approach
struct ValidatedRequest {
// Can only be constructed after passing RuleEngine validation
inner: Request>,
}

impl ValidatedRequest {
// Private constructor - only RuleEngine can create these
fn new(request: Request, validation_result: RuleEvaluation) -> Result {
// Ensure all headers match validated URL
// Apply any security transformations
}
}

// Upstream client only accepts ValidatedRequest
fn send_to_upstream(request: ValidatedRequest) -> Result {
// Impossible to send unvalidated requests
}
```

## Benefits
1. **Compile-time safety** - Makes it impossible to accidentally forward unvalidated or modified requests
2. **Clear security boundaries** - Type system enforces that all upstream requests go through validation
3. **Prevents regression** - New code can't accidentally bypass security checks
4. **Self-documenting** - Types make security requirements explicit

首先定位代理请求处理的入口点和RuleEngine，然后跟踪请求如何经过验证、转换并发送到上游。issue未指定任何文件或测试；完成的标准是，上游转发只接受一种能够在验证后创建的类型，并覆盖Host-header及相关的bypass检查。

由索引模型根据 Issue 内容生成。