feat(video): add OpenRouter and Vertex AI (Veo) video generation

Video generation was xAI-only. Adds an adapter layer under
open-sse/handlers/videoProviders/ so /v1/videos/* can target OpenRouter or
Google Cloud credentials. A provider with no adapter keeps the exact previous
behaviour (raw body to {baseUrl}/{action}, poll {baseUrl}/{id}, verbatim
passthrough), so the xAI path is unchanged.

- openrouter: async job shape identical to xAI; creation POSTs to the /videos
  collection root (no /generations suffix) and the registry HTTP-Referer /
  X-Title headers are applied. Bodies pass through verbatim.
- vertex: two-way translation, since Veo does not speak the OpenAI-ish videos
  shape. create -> :predictLongRunning { instances[], parameters{} }, poll ->
  :fetchPredictOperation (Veo has no REST GET poll). The operation resource
  name is base64url-encoded into the job id so GET /v1/videos/{id} stays a
  flat path. Access tokens are minted from Service Account JSON via the
  existing refreshVertexToken; raw API keys are rejected up front. The
  operation response maps back onto the { id, status, video, videos } shape
  clients already poll.
- videoCore: the request plan is rebuilt per attempt, so the 401 -> refresh
  once -> retry once path picks up the refreshed token. Adapter validation
  errors return 400 before any upstream call, so a malformed request can never
  create a billable job.
- videoGeneration: GET /v1/videos/{id} resolves the provider from the pinned
  x-connection-id connection, then ?provider=, then falls back to the xAI
  default.
- registry: openrouter and vertex gain videoConfig, the video serviceKind and
  video-kind models (Veo 3.1 / 3 / 2, Sora 2 Pro, Seedance 2.0).

open-sse/handlers/videoProviders/index.js

@@ -0,0 +1,13 @@
+// Video provider adapters.
+//
+// Default (no adapter) = xAI shape: raw body forwarded to {baseUrl}/{action},
+// polled at {baseUrl}/{id}, upstream JSON passed through verbatim.
+// A provider only needs an adapter when its wire format differs from that.
+import openrouter from "./openrouter.js";
+import vertex from "./vertex.js";
+
+const ADAPTERS = { openrouter, vertex };
+
+export function getVideoAdapter(provider) {
+  return ADAPTERS[provider] || null;
+}