[ty] Handle cycles in function decorator inference (#25475)

## Summary

Handle recursive `function_known_decorators` queries by falling back to
an empty decorator inference result while Salsa resolves the cycle. This
prevents a Python 3.14 descriptor annotation with an overloaded
`__get__` method from panicking and preserves conservative `Unknown`
behavior during cycle recovery.

Closes https://github.com/astral-sh/ty/issues/3593.