fix: allow loopback relay in proxy config test

[Itsusinn]

Summary

Fixes the CI failure in the proxy_config integration test (test_client_proxy_configuration) that has broken every test-running job since commit 0cdb82a ("tests: return error", run 806+), including the linked run 810.

Root cause

The test starts a tuic-server without overriding experimental. ExperimentalConfig defaults to drop_loopback = true / drop_private = true (crates/tuic-server/src/config.rs), and TuicRouter::do_route rejects loopback/private targets before consulting the ACL (crates/tuic-server/src/wind_adapter.rs). So the server always refused to connect to the 127.0.0.1 echo server, the SOCKS5 relay chain never succeeded, and the check was silently soft-logged ("may be expected") until 0cdb82a turned it into a hard assert!.

All other integration tests (quiche_server_config / quinn_server_config in crates/tuic-tests/src/lib.rs) explicitly set drop_loopback: false, drop_private: false — this test was the lone outlier (its unused ExperimentalConfig import, masked by #![allow(unused_imports)], hints the override was intended).

Fix

Mirror the shared helpers: set experimental: ExperimentalConfig { drop_loopback: false, drop_private: false } on the test's server config.

Verification

• CI history: run 805 (55016ce) ✅ → run 806 (0cdb82a) ❌; the only behavioral change is this assert.
• Locally built and ran the test on i686-pc-windows-msvc and x86_64-pc-windows-msvc (quiche/BoringSSL included):
  • Without the fix: Bi stream error: connection rejected: loopback address rejected: 127.0.0.1:...
  • With the fix: server accepts + authenticates, TCP connect forwarded, echo server receives and echoes the data.
• cargo +nightly fmt --all -- --check passes; diff is 7 insertions in one file.
• Note: full E2E green is best confirmed by CI on this PR (the local host has an unrelated environment quirk in the TUIC return path that also fails the CI-passing quinn_zero_rtt/concurrency tests locally; CI at run 805 was green on all platforms).

Follow-up (not in scope)

tuic-client's relay.proxy (SOCKS5 proxy for the client's QUIC connection) is parsed in config.rs but never consumed — the client always connects directly, so this test exercises a plain relay chain rather than an actual proxied connection. Implementing the feature would be a separate change.

Assisted-by: Reasonix:deepseek-v4-flash