ruff

f80f6b77 - [ty] Fix bug where diagnostics could disappear after opening an external file

[ty] Fix bug where diagnostics could disappear after opening an external file This happened whenever: * Diagnostic mode was set to `openFilesOnly` * One opened and then closed an external file (or caused the LSP client to send notifications that a file was closed, which may happen even when the end user doesn't actually open or close a file) When this happened, the entire open file set was cleared. This in turn resulted in `should_check_file` returning `false` for any file previously in the open file set. And thus, we'd never get any diagnostics. Looking at our revision history, it seems like this bug has been present for a long time? I'm not sure. However, this bug doesn't apply when the diagnostic mode is set to `workspace`, which might explain why it defied reproduction. I've also included a regression test and some extra TRACE level logs that might help diagnose these sorts of problems in the future. Fixes https://github.com/astral-sh/ty-vscode/issues/342