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/agent/src/context_picker/completion_provider.rs

@@ -765,7 +765,7 @@ impl CompletionProvider for ContextPickerCompletionProvider {
 
         let snapshot = buffer.read(cx).snapshot();
         let source_range = snapshot.anchor_before(state.source_range.start)
-            ..snapshot.anchor_before(state.source_range.end);
+            ..snapshot.anchor_after(state.source_range.end);
 
         let thread_store = self.thread_store.clone();
         let text_thread_store = self.text_thread_store.clone();