test: add unit tests for proxy resolution module

- 28 test cases covering resolveProxyUrl, getProxyDispatcher, proxyFetch
- System env: HTTPS_PROXY, HTTP_PROXY, SOCKS_PROXY, SOCKS5_PROXY
- DEEPCODE_ prefixed vars, lowercase fallback
- settings.json: user-level and project-level env config
- Priority: user < project < system
- NO_PROXY: exact match, wildcard *, subdomain (.example.com)
- Dispatcher caching verification
- proxyFetch fallback to global fetch and dispatcher injection

Author: kiangz