refactor: remove duplicate gpt-5.5-pro thinking-policy test

When #3192 renamed `gpt-5.4-pro` → `gpt-5.5-pro` across
`policy.test.ts`, it accidentally introduced a third
`returns medium/high/xhigh for gpt-5.5-pro` test that is
byte-identical to the renamed first occurrence (the remaining
two unique tests are the bare-prefix and `with version suffix`
variants; the deleted block had no version suffix and no gateway
prefix). Drop the duplicate so the suite has one canonical
no-suffix test, one mux-gateway test, and one version-suffix
test.

Behavior-preserving — the remaining `getThinkingPolicyForModel`
coverage for `gpt-5.5-pro` is unchanged. 63 / 63 tests in
\`policy.test.ts\` continue to pass.

Author: mux-bot

src/common/utils/thinking/policy.test.ts

@@ -76,10 +76,6 @@ describe("getThinkingPolicyForModel", () => {
     ]);
   });
 
-  test("returns medium/high/xhigh for gpt-5.5-pro", () => {
-    expect(getThinkingPolicyForModel("openai:gpt-5.5-pro")).toEqual(["medium", "high", "xhigh"]);
-  });
-
   test("returns 5 levels including xhigh for gpt-5.3-codex", () => {
     expect(getThinkingPolicyForModel("openai:gpt-5.3-codex")).toEqual([
       "off",