[ty] Fix subtraction overflow bug

PR #21549 introduced a subtle overflow bug that seemed impossible, but
can empirically happen. This PR fixes it by saturating to zero.

I did try to write a regression test for this, but couldn't manage it.
Instead, I'll attach before-and-after screen recordings.

crates/ruff_text_size/src/size.rs

@@ -106,6 +106,25 @@ impl TextSize {
     pub fn checked_sub(self, rhs: TextSize) -> Option<TextSize> {
         self.raw.checked_sub(rhs.raw).map(|raw| TextSize { raw })
     }
+
+    /// Saturating addition. Returns maximum `TextSize` if overflow occurred.
+    #[inline]
+    #[must_use]
+    pub fn saturating_add(self, rhs: TextSize) -> TextSize {
+        TextSize {
+            raw: self.raw.saturating_add(rhs.raw),
+        }
+    }
+
+    /// Saturating subtraction. Returns minimum `TextSize` if overflow
+    /// occurred.
+    #[inline]
+    #[must_use]
+    pub fn saturating_sub(self, rhs: TextSize) -> TextSize {
+        TextSize {
+            raw: self.raw.saturating_sub(rhs.raw),
+        }
+    }
 }
 
 impl From<u32> for TextSize {

crates/ty_ide/src/completion.rs

@@ -1341,7 +1341,7 @@ fn is_in_definition_place(
 /// E.g. naming a parameter, type parameter, or `for` <name>).
 fn is_in_variable_binding(parsed: &ParsedModuleRef, offset: TextSize, typed: Option<&str>) -> bool {
     let range = if let Some(typed) = typed {
-        let start = offset - typed.text_len();
+        let start = offset.saturating_sub(typed.text_len());
         TextRange::new(start, offset)
     } else {
         TextRange::empty(offset)