fix(release): drain two v3.8.50 base-reds (volcengine vision metadata, antigravity BYOP contract)

Two unrelated real reds on the release tip:

* fix(providers): flag MiniMax M3 as multimodal on the Volcengine Ark plans.
  d732cf615 ("feat(volcengine): add Ark plan providers") added
  volcengine-agent-plan/minimax-m3 and volcengine-coding-plan/minimax-m3
  without supportsVision, breaking the LEDGER-4 invariant that every
  minimax-m3 registry entry except PromptQL (text-only upstream) is flagged
  multimodal. Every other provider carrying the model (opencode-zen,
  opencode-go, bazaarlink, ollama-cloud, codebuddy-cn, trae) sets it.
  Registry metadata defect, not a stale test.

* test(antigravity): align the empty-projectId onboarding test to the
  contract shipped by #11284/#11358 (6de542b9b). That change made an
  onboardUser 200 whose body carries NO cloudaicompanionProject mean Google
  BYOP — no project was created and none ever will be — so it short-circuits
  before the retry loadCodeAssist. The older test still mocked onboardUser
  with the bare { done: true } BYOP shape while asserting the retry path, so
  it pinned a contract that was deliberately moved. The mock now returns a
  real onboarding-success body; every assertion is kept, and the id in the
  onboard body deliberately differs from the expected one so the test still
  proves the projectId came from the retry discovery.

Refs #11284

tests/unit/antigravity-oauth-postexchange-nonblocking.test.ts

@@ -152,7 +152,12 @@ test("postExchange attempts onboarding when projectId is empty and returns disco
     }
     if (u.includes("onboardUser")) {
       onboardUserCalled = true;
-      return jsonRes({ done: true });
+      // Real onboarding success: the body carries a cloudaicompanionProject.
+      // #11284 made that the discriminator — a 200 WITHOUT one is Google BYOP
+      // (no project was created, none ever will be) and short-circuits before
+      // the retry. The id here is deliberately NOT the expected one, so the
+      // assertion below still proves the value came from the retry discovery.
+      return jsonRes({ done: true, cloudaicompanionProject: "onboard-body-project" });
     }
     return jsonRes({});
   }) as typeof fetch;