[ty] Fix handling of non-Python text documents
In #22449, I added a check to our "did open" handler to effectively
ignore notifications for text documents that we were sure weren't
Python. This was meant to fix a case where we could return diagnostics
for non-Python files, which was undesirable.
However, it seems like that might have been too big of a hammer. It
seems like we might still want to track non-Python text files in our
index but not our project. Otherwise subsequent requests regarding that
non-Python file result in log messages saying that ty doesn't know about
the file. i.e., a state synchronization issue.
Addresses https://github.com/astral-sh/ruff/pull/23121#discussion_r2882639788
