[MCP] MCP Inspector warns about incompatible JSON Schemas / Change type hinting for nullable type

Since recently MCP Inspector warns about the produced schema type hints:

```
`type` is an array (["string","null"]). The array form is legal JSON Schema, but several MCP clients read `type` as a single string and either reject the tool or drop the constraint.

Fix: Split it into `anyOf` branches, each with a single `type` — `{"anyOf": [{"type": "string"}, {"type": "null"}]}`. (Making the property optional instead is a different contract: absent is not the same as `null`.)
```

Like the warning message says, the behavior of the API Platform here is not necessarily wrong, but changing the way how these type hints are declared might ensure wider compatibility.