fix(gateway): treat a blank GATEWAY_HOST/GATEWAY_PORT as unset (#5801)

os.getenv's default only covers an unset variable, so `docker run -e
GATEWAY_PORT=` — or `GATEWAY_PORT=${PORT}` with PORT unset — reached
int("") and aborted the create_app() import that both entry points run.
Blank now falls back to the declared default, while "0" stays a real port.

backend/app/gateway/config.py

@@ -19,8 +19,9 @@ def get_gateway_config() -> GatewayConfig:
     global _gateway_config
     if _gateway_config is None:
         _gateway_config = GatewayConfig(
-            host=os.getenv("GATEWAY_HOST", "0.0.0.0"),
-            port=int(os.getenv("GATEWAY_PORT", "8001")),
+            # A blank means "unset" (`-e GATEWAY_PORT=`, or `${PORT}` with PORT unset); int("") would abort the create_app() import
+            host=os.getenv("GATEWAY_HOST") or "0.0.0.0",
+            port=int(os.getenv("GATEWAY_PORT") or "8001"),
             enable_docs=os.getenv("GATEWAY_ENABLE_DOCS", "true").lower() == "true",
         )
     return _gateway_config

backend/tests/test_gateway_docs_toggle.py

@@ -78,6 +78,31 @@ def test_enable_docs_unexpected_value_disables():
             assert config.enable_docs is False, f"Expected False for GATEWAY_ENABLE_DOCS={value}"
 
 
+def test_blank_host_and_port_fall_back_to_defaults():
+    """A blank GATEWAY_HOST/GATEWAY_PORT means unset, not a crash or an empty bind address.
+
+    ``docker run -e GATEWAY_PORT=`` and ``environment: [GATEWAY_PORT=${PORT}]`` with PORT
+    unset both reach us as an empty string, and create_app() reads this at import time.
+    """
+    with patch.dict(os.environ, {"GATEWAY_HOST": "", "GATEWAY_PORT": ""}):
+        _reset_gateway_config()
+        from app.gateway.config import get_gateway_config
+
+        config = get_gateway_config()
+        assert config.port == 8001
+        assert config.host == "0.0.0.0"
+
+
+def test_zero_port_is_not_treated_as_unset():
+    """GATEWAY_PORT=0 asks the OS for a free port, so it must survive the blank fallback."""
+    with patch.dict(os.environ, {"GATEWAY_PORT": "0"}):
+        _reset_gateway_config()
+        from app.gateway.config import get_gateway_config
+
+        config = get_gateway_config()
+        assert config.port == 0
+
+
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