fix(gateway): load streaming config from nested gateway.streaming key

`hermes config set gateway.streaming.*` writes the streaming block
nested under a `gateway:` key in config.yaml, but the config loader
only checked for a top-level `streaming:` key — silently ignoring
the nested variant.

Fall back to `yaml_cfg['gateway']['streaming']` when the top-level
key is absent, matching the pattern already used for other nested
config sections.

Closes #25676

tests/test_gateway_streaming_nested_config.py

@@ -0,0 +1,46 @@
+"""Regression test for #25676 — nested gateway.streaming config must be loaded."""
+from pathlib import Path
+from unittest.mock import patch, MagicMock
+import json
+
+import pytest
+import yaml
+
+
+def _load_with_yaml_dict(yaml_dict: dict):
+    """Patch filesystem so load_gateway_config() sees *yaml_dict* as config.yaml."""
+    from gateway.config import load_gateway_config
+
+    fake_home = Path("/tmp/fake_hermes_home_25676")
+
+    def fake_exists(self):
+        return str(self).endswith("config.yaml")
+
+    with patch("gateway.config.get_hermes_home", return_value=fake_home), \
+         patch.object(Path, "exists", fake_exists), \
+         patch("builtins.open", create=True) as mock_file:
+        mock_file.return_value.__enter__ = lambda s: s
+        mock_file.return_value.__exit__ = MagicMock(return_value=False)
+        with patch("yaml.safe_load", return_value=yaml_dict):
+            return load_gateway_config()
+
+
+class TestStreamingConfigNested:
+    def test_top_level_streaming(self):
+        cfg = _load_with_yaml_dict({"streaming": {"enabled": True, "transport": "draft"}})
+        assert cfg.streaming.enabled is True
+        assert cfg.streaming.transport == "draft"
+
+    def test_nested_gateway_streaming(self):
+        """Regression for #25676."""
+        cfg = _load_with_yaml_dict({"gateway": {"streaming": {"enabled": True, "transport": "draft"}}})
+        assert cfg.streaming.enabled is True
+        assert cfg.streaming.transport == "draft"
+
+    def test_top_level_takes_precedence(self):
+        cfg = _load_with_yaml_dict({
+            "streaming": {"enabled": True, "transport": "edit"},
+            "gateway": {"streaming": {"enabled": False, "transport": "draft"}},
+        })
+        assert cfg.streaming.enabled is True
+        assert cfg.streaming.transport == "edit"