Improve _load_required_env error message clarity

Distinguish between missing argument and missing environment variable
in the RuntimeError message to speed up debugging.

src/summary_mcp/workflows/freshrss_pipeline.py

@@ -53,10 +53,14 @@ def _load_json(path: Path) -> dict[str, Any]:
 
 def _load_required_env(name: str, value: str | None) -> str:
     # 优先使用传入的 value，否则读取同名环境变量；两者均缺失时抛出 RuntimeError
-    resolved = value or os.environ.get(name)
-    if not resolved:
-        raise RuntimeError(f"Missing required value: {name}.")
-    return resolved
+    if value:
+        return value
+    env_value = os.environ.get(name)
+    if env_value:
+        return env_value
+    raise RuntimeError(
+        f"Missing required value '{name}': not passed as argument and not set as environment variable."
+    )
 
 
 def default_output_dir() -> Path: