fix(feishu-auth): use correct 3rd return value from admin API GET in TEST 14

The test helper t() returns (code, message, body), so the GET response
body is the third return value. Using only two return values caused
json.decode to receive the message string instead of the JSON body,
resulting in nil dereference.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: AlinsRan

t/plugin/feishu-auth.t

@@ -657,10 +657,10 @@ apisix:
             ngx.sleep(0.1)
 
             -- Admin API should return decrypted values
-            local code2, body2 = t('/apisix/admin/routes/1', ngx.HTTP_GET)
+            local code2, _, res2 = t('/apisix/admin/routes/1', ngx.HTTP_GET)
             assert(code2 == 200, "admin get route failed: " .. tostring(code2))
-            local data = json.decode(body2)
-            local conf = data.value.plugins["feishu-auth"]
+            res2 = json.decode(res2)
+            local conf = res2.value.plugins["feishu-auth"]
             ngx.say("admin secret_fallbacks[1]: ", conf.secret_fallbacks[1])
 
             -- etcd should store encrypted values