fix(providers): disable proxy for local endpoints, respect env proxy for cloud

When the host has HTTP_PROXY / HTTPS_PROXY / ALL_PROXY set, httpx routes
all traffic through the proxy — including requests to localhost or LAN
addresses that the proxy typically cannot reach.  This breaks local model
servers (Ollama, llama.cpp, vLLM) silently.

- Local endpoints: pass transport=httpx.AsyncHTTPTransport(proxy=None)
  so proxy env vars are ignored for local traffic.
- Cloud endpoints: pass trust_env=True so corporate/VPN proxies work
  without explicit configuration.

Fixes #4366

tests/providers/test_openai_compat_timeout.py

@@ -16,7 +16,10 @@ async def test_openai_compat_provider_defers_sdk_client_until_first_use() -> Non
 
     kwargs = mock_async_openai.call_args.kwargs
     _assert_openai_compat_timeout(kwargs["timeout"])
-    assert kwargs["http_client"] is None
+    # Cloud endpoints get an httpx client with trust_env=True to respect
+    # proxy environment variables (HTTP_PROXY, HTTPS_PROXY, ALL_PROXY).
+    assert kwargs["http_client"] is not None
+    assert kwargs["http_client"]._trust_env is True
 
 
 async def test_openai_compat_provider_sets_timeout_on_local_http_client() -> None: