Use actual function name in get_schema_from_signature

[leloykun]

As discussed with @eitanturok in #863

---

Currently, get_schema_from_signature converts the following function:

def test_add(a: int, b: int | None = None):
    if b is None:
        return a
    return a + b

to the schema:

{
    "properties": {
        "a": {"title": "A", "type": "integer"},
        "b": {"title": "B", "type": "integer"},
    },
    "required": ["a"],
    "title": "Arguments",
    "type": "object",
}

The issue is that the title of the schema is always "Arguments", regardless of the name of the function. And in some cases, this breaks the performance of the model as discussed by @eitanturok

[leloykun]

now a duplicate of #875