fix(tui): resolve wrong provider prefix when session has model without modelProvider (#25874)

Merged via /review-pr -> /prepare-pr -> /merge-pr.

Prepared head SHA: f0953a7284
Co-authored-by: lbo728 <72309817+lbo728@users.noreply.github.com>
Co-authored-by: gumadeiras <5599352+gumadeiras@users.noreply.github.com>
Reviewed-by: @gumadeiras

src/gateway/server.sessions.gateway-server-sessions-a.test.ts

@@ -202,6 +202,8 @@ describe("gateway server sessions", () => {
         main: {
           sessionId: "sess-main",
           updatedAt: recent,
+          modelProvider: "anthropic",
+          model: "claude-sonnet-4-6",
           inputTokens: 10,
           outputTokens: 20,
           thinkingLevel: "low",
@@ -456,11 +458,13 @@ describe("gateway server sessions", () => {
     const reset = await rpcReq<{
       ok: true;
       key: string;
-      entry: { sessionId: string };
+      entry: { sessionId: string; modelProvider?: string; model?: string };
     }>(ws, "sessions.reset", { key: "agent:main:main" });
     expect(reset.ok).toBe(true);
     expect(reset.payload?.key).toBe("agent:main:main");
     expect(reset.payload?.entry.sessionId).not.toBe("sess-main");
+    expect(reset.payload?.entry.modelProvider).toBe("anthropic");
+    expect(reset.payload?.entry.model).toBe("claude-sonnet-4-6");
     const filesAfterReset = await fs.readdir(dir);
     expect(filesAfterReset.some((f) => f.startsWith("sess-main.jsonl.reset."))).toBe(true);