Input validators crash fix

[darrenburns]

Supplying a list to Input.validate_on causes a crash starting from v0.76.0.

|/textual/widgets/_input.py:304 in __init__
...
│   303 │   │   self.validate_on: set[str] = (                                               │
│ ❱ 304 │   │   │   (_POSSIBLE_VALIDATE_ON_VALUES & cast("set[str]", validate_on))           │
│   305 │   │   │   if validate_on is not None                                               │
│   306 │   │   │   else _POSSIBLE_VALIDATE_ON_VALUES                                        │
│   307 │   │   )

TypeError: RequestOptions() compose() method returned an invalid result; unsupported operand
type(s) for &: 'set' and 'list'