[ty] Infer generic elements from gradual tuples (#28637)
A gradual tuple `tuple[Any, ...]` is assumed to materialize to the
correct length when assigned to a fixed-length tuple, and the gradual
element type should contribute to generic inference instead of falling
back to `Unknown`.
```py
from typing import Any
def _(values: list[tuple[Any, ...]]):
reveal_type(dict(values)) # main: dict[Unknown, Unknown], now: dict[Any, Any]
```
Follow up to https://github.com/astral-sh/ruff/pull/28588.
