#8571: fix: support system proxy in web_fetch SSRF dispatcher

### Description This Pull Request fixes an issue where the `web_fetch` tool fails in restricted network environments that require a system proxy (e.g., Clash, corporate proxies). ### Problem The `createPinnedDispatcher` function in `src/infra/net/ssrf.ts` was hardcoded to use a standard `undici.Agent`. This agent performs direct IP connections to prevent SSRF but ignores the standard Node.js proxy environment variables (`HTTPS_PROXY`, `HTTP_PROXY`). In environments where direct outgoing connections are blocked, `web_fetch` would consistently fail with `TypeError: fetch failed`. ### Solution Updated `createPinnedDispatcher` to check for proxy environment variables. If a proxy is configured, it now returns a `undici.ProxyAgent`, allowing `web_fetch` to respect the system proxy while maintaining the required dispatcher architecture. ### Changes - Updated imports in `src/infra/net/ssrf.ts` to include `ProxyAgent` from `undici`. - Modified `createPinnedDispatcher` to prefer `ProxyAgent` if `HTTPS_PROXY` or `HTTP_PROXY` is set. Verified this fix on a production instance behind a local proxy. <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> This PR updates `src/infra/net/ssrf.ts` to honor system proxy environment variables in `createPinnedDispatcher` by returning an `undici.ProxyAgent` when a proxy is configured, otherwise falling back to the existing pinned `undici.Agent` (with `connect.lookup` set to the pinned resolver). This is intended to make `web_fetch` work in restricted egress environments that require a proxy. Key concern: the proxy branch changes the security model—requests will be routed through the proxy and DNS/connection pinning via `pinned.lookup` no longer applies to the ultimate destination—so SSRF protections may be weakened unless destination validation is enforced elsewhere in the fetch path. <h3>Confidence Score: 2/5</h3> - This PR is not safe to merge as-is if SSRF protections must hold in proxy-required deployments. - While the change likely fixes connectivity behind proxies, switching to `ProxyAgent` bypasses the pinned DNS/connection behavior that existed specifically to mitigate SSRF; without additional destination enforcement, this can materially weaken security in environments where the proxy can reach internal networks. Minor env-var coverage issues also remain (missing `https_proxy`). - src/infra/net/ssrf.ts (proxy branch in createPinnedDispatcher) <!-- greptile_other_comments_section --> _{(2/5) Greptile learns from your feedback when you react with thumbs up/down!} <!-- /greptile_comment -->