feat(providers): add max thinking level for gpt-5.6-sol (#2500)

Expose max in the Codex thinking dropdown for gpt-5.6-sol only (maps to
xhigh on wire; live probe rejected ultra). Include custom/kilo models
when computing provider thinking options so manually added gpt-5.6-sol
contributes its max level.

Co-authored-by: Cursor <cursoragent@cursor.com>

tests/unit/thinking-levels-gpt56-sol.test.js

@@ -0,0 +1,21 @@
+import { describe, it, expect } from "vitest";
+import { getThinkingLevels } from "../../open-sse/providers/thinkingLevels.js";
+
+describe("getThinkingLevels", () => {
+  it("adds max for gpt-5.6-sol on codex", () => {
+    const levels = getThinkingLevels("codex", "gpt-5.6-sol");
+    expect(levels).toContain("max");
+    expect(levels).toContain("xhigh");
+    expect(levels).not.toContain("ultra");
+  });
+
+  it("does not add max for other codex models", () => {
+    const levels = getThinkingLevels("codex", "gpt-5.3-codex");
+    expect(levels).toEqual(["low", "medium", "high", "xhigh"]);
+  });
+
+  it("does not add max for other gpt-5.6 models", () => {
+    const levels = getThinkingLevels("codex", "gpt-5.5");
+    expect(levels || []).not.toContain("max");
+  });
+});