fix(pvisor): separate network-only proxying from Gateway routing

[lizhicui]

Summary

• keep allow-listed absolute-URI requests such as /v1/responses on the transparent proxy path when Gateway is off, preventing network-only pVisor runs from failing with no route for model; reject relative Gateway requests with HTTP 400 in this mode
• stop injecting LLM SDK base URLs in network-only runs while retaining HTTP proxy configuration, existing CLI flags, and enabled/standalone Gateway routing behavior
• add two regressions covering network-only forwarding without model routes and enabled Gateway model forwarding/rewriting

Validation

• RUSTC_WRAPPER= cargo nextest run --locked -p persisting-gateway --test network_policy_http --filter-expr 'test(e2e_absolute_uri_llm_keeps_gateway_model_routing) | test(e2e_network_only_allowlist_forwards_absolute_llm_uri)' — both regressions passed
• RUSTC_WRAPPER= just test persisting-overlaynet — 66 passed
• RUSTC_WRAPPER= just test persisting-agentctl — 27 passed
• Linux amd64 release binary built with cargo zigbuild; the build required a temporary fuser build-script host/target detection workaround, restored after compilation and excluded from this PR
• Debian bookworm slim container: version, Run help, and a basic network-enabled Run smoke check succeeded; namespace isolation was unavailable and reported a best-effort fallback
• pVisor package tests remain blocked on Intel macOS by libkrun's kvm_bindings dependency; container HTTP allow/deny end-to-end verification is incomplete