fix(providers): remove custom cloud httpx client, let SDK handle proxy defaults

chengyongru reviewed #4367 and identified that the cloud  branch
created a bare httpx.AsyncClient that lacked the SDK's default settings
(follow_redirects, connection pool limits). Since the SDK's
DefaultAsyncHttpxClient already has trust_env=True and proper defaults,
the simplest fix is to let http_client stay None for cloud endpoints.

Also updated the test to match the new behavior (http_client is None).

nanobot/providers/openai_compat_provider.py

@@ -417,14 +417,9 @@ class OpenAICompatProvider(LLMProvider):
                 timeout=timeout_s,
                 transport=httpx.AsyncHTTPTransport(proxy=None, limits=_local_limits),
             )
-        else:
-            # Cloud endpoints: respect proxy environment variables
-            # (HTTP_PROXY, HTTPS_PROXY, ALL_PROXY, NO_PROXY) so corporate
-            # or VPN proxies work without explicit configuration.
-            http_client = httpx.AsyncClient(
-                timeout=timeout_s,
-                trust_env=True,
-            )
+        # else: http_client stays None → SDK creates DefaultAsyncHttpxClient
+        # which already reads proxy env vars via trust_env=True, has proper
+        # connection limits, and follows redirects.
         self._client = AsyncOpenAI(
             api_key=self._api_key_for_client,
             base_url=self._effective_base,