All HTTP clients previously used http.DefaultTransport which silently respects
HTTP_PROXY/HTTPS_PROXY env vars, allowing credentials to transit through
untrusted proxies. This adds a proxy detection warning and an opt-out switch
(LARK_CLI_NO_PROXY=1) so security-sensitive users can disable proxy entirely.
- Redact proxy credentials in warning output (handles both scheme-prefixed and bare URL formats)
- Suppress warning when LARK_CLI_NO_PROXY is already set
- Use FallbackTransport singleton for nil-Base fallback paths to preserve connection pooling
- Emit proxy warning on both HTTP client and Lark SDK client paths
Change-Id: Ibed7d0470409c73fbd42bccac6673f9fc5e87a83
