Increase timeout for process_response/3 to fix flaky tests

`changes_async_test.exs` become unstable when running Elixir tests
with `OTP 27.3.4.3` and `Elixir 1.18.4`. Tests are passing when
running against `OTP 27.3.4.2` and `OTP 27.3.4.1`.

Increate timeout for `process_response/3` to mediate it.

Error log:

```log
[2025-10-13T21:10:06.495Z]   1) test continuous filtered changes with doc ids (ChangesAsyncTest)
[2025-10-13T21:10:06.495Z]      src/couchdb/test/elixir/test/changes_async_test.exs:309
[2025-10-13T21:10:06.495Z]      Expected truthy, got false
[2025-10-13T21:10:06.495Z]      code: assert Enum.member?(changes_ids, "doc3")
[2025-10-13T21:10:06.495Z]      arguments:
[2025-10-13T21:10:06.495Z]
[2025-10-13T21:10:06.495Z]          # 1
[2025-10-13T21:10:06.495Z]          ["doc1"]
[2025-10-13T21:10:06.495Z]
[2025-10-13T21:10:06.495Z]          # 2
[2025-10-13T21:10:06.495Z]          "doc3"
[2025-10-13T21:10:06.495Z]
[2025-10-13T21:10:06.495Z]      stacktrace:
[2025-10-13T21:10:06.495Z]        src/couchdb/test/elixir/test/changes_async_test.exs:338: (test)
```

Author: jiahuili430

test/elixir/test/changes_async_test.exs

@@ -394,7 +394,7 @@ defmodule ChangesAsyncTest do
     end
   end
 
-  defp process_response(id, chunk_parser, timeout \\ 1000) do
+  defp process_response(id, chunk_parser, timeout \\ 3000) do
     receive do
       %HTTPotion.AsyncChunk{id: ^id} = msg ->
         chunk_parser.(msg)