test(pixel): pin the runtime gateway admission and SSRF-guard contract

[tang-vu]

Summary

Adds ods/tests/pixel_inference/test_runtime_gateway_policy.py — boundary-policy coverage for ods/bin/pixel_provider/runtime_gateway.py, the turn-scoped OpenAI transport every Pixel generation call crosses. test_provider_runtime.py already covers failover/streaming mechanics; this pins the admission and SSRF-guard surface it did not.

Why this matters

This is the last process boundary before a turn leaves the device: request validation decides what may reach a provider, pinned_target is the DNS-rebinding/SSRF guard, and the session policy (single-flight, terminal-on-failure, call limit, cooldown) is what prevents SDK retries from multiplying provider attempts.

What is tested (56 cases)

• validate_request — envelope allowlist, model=='ods/pixel', non-empty message list, bool stream, n==1, mutually exclusive max_tokens/max_completion_tokens; all six message roles; external-media gate — only data:image/{png,jpeg,webp};base64 URLs pass, every remote/foreign-scheme URL raises external-media-not-allowed; tools must be type:'function'; chat_template_kwargs limited to {enable_thinking: bool}.
• pinned_target — loopback HTTP allowed with X-ODS-Pixel-Route-Hop/Host headers; link-local, private-LAN-over-http, unspecified, v4-mapped public, v6 link-local, and v6 multicast all raise unsafe-provider-address; HTTPS public allowed with sni_hostname extension; IPv6 loopback bracketed correctly.
• Endpoint policy — missing/wrong Authorization → 401; X-ODS-Pixel-Route-Hop → 409 provider-route-cycle; transient upstream exhaustion → provider-attempts-exhausted then provider-session-stopped on the next call; non-JSON / wrong-shape upstream bodies → invalid-provider-response; non-SSE content-type on stream:true → invalid-provider-stream; missing data: [DONE] → provider-stream-interrupted and a terminal session (no client retry can splice a backup); oversized request body rejected; response headers carry provider id, revision, request id, Cache-Control: no-store.

Overlap check

Searched runtime_gateway, "runtime gateway". #5647 (open, roshangupta00750) adds stream-tail chunk-boundary [DONE] detection + tests in test_provider_runtime.py — different file, compatible semantics (my no-[DONE] → interrupted assertion holds under both). #5607/#5608 are dashboard-api/pixel-edge stream handling. My own #5659/#5661/#5663/#5665/#5668/#5669/#5671/#5674/#5675/#5676/#5677/#5681/#5686/#5687/#5688 touch other modules.

Validation

$ pytest ods/tests/pixel_inference/test_runtime_gateway_policy.py -x -q
56 passed in 4.39s

Test-only change; revert is a single file deletion.