fix(llm): use httpx.Timeout read-inactivity for per-turn model timeout

tests/test_inputs.py

@@ -164,7 +164,8 @@ def test_make_model_settings_sets_request_timeout() -> None:
         request_timeout=300.0,
     )
 
-    assert settings.extra_args == {"timeout": 300.0}
+    assert settings.extra_args is not None
+    assert settings.extra_args["timeout"].read == 300.0
 
 
 def test_make_model_settings_omits_timeout_when_unset() -> None:
@@ -182,4 +183,5 @@ def test_make_model_settings_timeout_survives_reasoning_resolve() -> None:
         request_timeout=120.0,
     )
 
-    assert settings.extra_args == {"timeout": 120.0}
+    assert settings.extra_args is not None
+    assert settings.extra_args["timeout"].read == 120.0