test: update model-fallback and chat-headers test expectations

🤖 Generated with assistance of [OhMyOpenCode](https://github.com/code-yeongyu/oh-my-opencode)

src/plugin/chat-headers.test.ts

@@ -94,10 +94,10 @@ describe("createChatHeadersHandler", () => {
 
     await handler(
       {
-        sessionID: "ses_1",
+        sessionID: "ses_3",
         provider: { id: "github-copilot" },
         message: {
-          id: "msg_1",
+          id: "msg_3",
           role: "user",
         },
       },

src/plugin/event.model-fallback.test.ts

@@ -332,10 +332,10 @@ describe("createEventHandler - model fallback", () => {
 
     //#then - second fallback entry applied (chain advanced)
     expect(second.message["model"]).toEqual({
-      providerID: "anthropic",
-      modelID: "gpt-5.3-codex",
+      providerID: "opencode",
+      modelID: "kimi-k2.5-free",
     })
-    expect(second.message["variant"]).toBe("high")
+    expect(second.message["variant"]).toBeUndefined()
     expect(abortCalls).toEqual([sessionID, sessionID])
     expect(promptCalls).toEqual([sessionID, sessionID])
     expect(toastCalls.length).toBeGreaterThanOrEqual(0)