feat(proxy): centralize proxy handling with `proxyutil` package and enhance test coverage
- Added `proxyutil` package to simplify proxy handling across the codebase.
- Refactored various components (`executor`, `cliproxy`, `auth`, etc.) to use `proxyutil` for consistent and reusable proxy logic.
- Introduced support for "direct" proxy mode to explicitly bypass all proxies.
- Updated tests to validate proxy behavior (e.g., `direct`, HTTP/HTTPS, and SOCKS5).
- Enhanced YAML configuration documentation for proxy options.
- Added nil check for proxy user credentials to prevent potential nil pointer dereference.
- Enhanced authentication logic for SOCKS5 proxies in `proxy_helpers.go` and `proxy.go`.
- Added `newProxyAwareHTTPClient` to centralize proxy configuration with priority on `auth.ProxyURL` and `cfg.ProxyURL`.
- Integrated enhanced proxy support across executors for HTTP, HTTPS, and SOCKS5 protocols.
- Refactored redundant HTTP client initialization to use `newProxyAwareHTTPClient` for consistent behavior.
