Filing to re-open #29951 which either isn't fixed, or is fixed only for some output that I'm not seeing. I cannot find the kind of output referenced in #29951 (comment)

Does this issue occur when all extensions are disabled?: Yes

• VS Code Version: 1.82.0
• OS Version: macOS 13.4.1

Steps to Reproduce:

1. Write a task with a custom problemMatcher, like so:

    {
     "version": "2.0.0",
     "tasks": [
         {
     	    "label": "build-x",
     	    "type": "shell",
     	    "command": "npm",
     	    "args": [
     		    "run",
     		    "build",
     		    "--watch",
     		    "-w",
     		    "@lit-labs/x"
     	    ],
     	    "problemMatcher": [{
     		    "background": {
     			    "activeOnStart": true,
     			    "endsPattern": "Watching for file changes"
     		    }
     	    }, "$tsc-watch"],
     	    "isBackground": true,
     	    "presentation": {
     		    "reveal": "always"
     	    },
     	    "group": {
     		    "kind": "build",
     		    "isDefault": true
     	    }
         }
     ]
   }

2. See an error toast with the text: "There are task errors. See the output for details."
3. Click on "Show Output"
4. See the following buffer:

Error: the description can't be converted into a problem matcher:
{
    "background": {
        "activeOnStart": true,
        "endsPattern": "Watching for file changes"
    }
}

This error message doesn't say what's wrong with the problemMatcher (all fields are optional in the schema, so nothing is obvious to me).