Allow Nullable Required Model fields

[electrocucaracha]

Considering the following scenario:

@mcp.tool()
def echo_tool(id: int|None, text: str = Field(description="The text to echo")) -> str:
...

Which results in this input schema

{'properties': {'id': {'anyOf': [{'type': 'integer'}, {'type': 'null'}], 'title': 'Id'}, 'text': {'description': 'The text to echo', 'title': 'Text', 'type': 'string'}}, 'required': ['id', 'text'], 'title': 'echo_toolArguments', 'type': 'object'}

Where the id field is required but nullable at the same time.

For this scenario, the field needs to be interpreted as Python Optional field. This PR allows that case.