[ty] Track open files in the server (#19264)

## Summary

This PR updates the server to keep track of open files both system and
virtual files.

This is done by updating the project by adding the file in the open file
set in `didOpen` notification and removing it in `didClose`
notification.

This does mean that for workspace diagnostics, ty will only check open
files because the behavior of different diagnostic builder is to first
check `is_file_open` and only add diagnostics for open files. So, this
required updating the `is_file_open` model to be `should_check_file`
model which validates whether the file needs to be checked based on the
`CheckMode`. If the check mode is open files only then it will check
whether the file is open. If it's all files then it'll return `true` by
default.

Closes: astral-sh/ty#619

## Test Plan

### Before

There are two files in the project: `__init__.py` and `diagnostics.py`.

In the video, I'm demonstrating the old behavior where making changes to
the (open) `diagnostics.py` file results in re-parsing the file:


https://github.com/user-attachments/assets/c2ac0ecd-9c77-42af-a924-c3744b146045

### After

Same setup as above.

In the video, I'm demonstrating the new behavior where making changes to
the (open) `diagnostics.py` file doesn't result in re-parting the file:


https://github.com/user-attachments/assets/7b82fe92-f330-44c7-b527-c841c4545f8f

crates/ty_project/src/files.rs

@@ -18,7 +18,7 @@ use crate::{IOErrorDiagnostic, Project};
 /// The implementation uses internal mutability to transition between the lazy and indexed state
 /// without triggering a new salsa revision. This is safe because the initial indexing happens on first access,
 /// so no query can be depending on the contents of the indexed files before that. All subsequent mutations to
-/// the indexed files must go through `IndexedMut`, which uses the Salsa setter `package.set_file_set` to
+/// the indexed files must go through `IndexedMut`, which uses the Salsa setter `project.set_file_set` to
 /// ensure that Salsa always knows when the set of indexed files have changed.
 #[derive(Debug)]
 pub struct IndexedFiles {
@@ -280,7 +280,7 @@ mod tests {
 
         // Calling files a second time should not dead-lock.
         // This can e.g. happen when `check_file` iterates over all files and
-        // `is_file_open` queries the open files.
+        // `should_check_file` queries the open files.
         let files_2 = project.file_set(&db).get();
 
         match files_2 {