Fix anchor biases for completion replacement ranges (esp slash commands) (#32262)

Closes #32205

The issue was that in some places the end of the replacement range used
anchors with `Bias::Left` instead of `Bias::Right`. Before #31872
completions were recomputed on every change and so the anchor bias
didn't matter. After that change, the end anchor didn't move as the
user's typing. Changing it to `Bias::Right` to "stick" to the character
to the right of the cursor fixes this.

Release Notes:

- Fixes incorrect auto-completion of `/files` in text threads (Preview
Only)

crates/editor/src/editor.rs

@@ -5067,10 +5067,16 @@ impl Editor {
             return;
         }
 
+        let multibuffer_snapshot = self.buffer.read(cx).read(cx);
+
         // Typically `start` == `end`, but with snippet tabstop choices the default choice is
         // inserted and selected. To handle that case, the start of the selection is used so that
         // the menu starts with all choices.
-        let position = self.selections.newest_anchor().start;
+        let position = self
+            .selections
+            .newest_anchor()
+            .start
+            .bias_right(&multibuffer_snapshot);
         if position.diff_base_anchor.is_some() {
             return;
         }
@@ -5083,8 +5089,9 @@ impl Editor {
         let buffer_snapshot = buffer.read(cx).snapshot();
 
         let query: Option<Arc<String>> =
-            Self::completion_query(&self.buffer.read(cx).read(cx), position)
-                .map(|query| query.into());
+            Self::completion_query(&multibuffer_snapshot, position).map(|query| query.into());
+
+        drop(multibuffer_snapshot);
 
         let provider = match requested_source {
             Some(CompletionsMenuSource::Normal) | None => self.completion_provider.clone(),