[ENH] Optimize FTS (#5592)

## Description of changes

_Summarize the changes made by this PR._

- Improvements & Bug fixes
    - Replace usage of `get_range` with `get_prefix`
- New functionality
    - N/A

## Test plan

_How are these changes tested?_

- [ ] Tests pass locally with `pytest` for python, `yarn test` for js,
`cargo test` for rust

## Migration plan

_Are there any migrations, or any forwards/backwards compatibility
changes needed in order to make sure this change deploys reliably?_

## Observability plan

_What is the plan to instrument and monitor this change?_

## Documentation Changes

_Are all docstrings for user-facing APIs updated if required? Do we need
to make documentation changes in the_ [_docs
section](https://github.com/chroma-core/chroma/tree/main/docs/docs.trychroma.com)?_

Author: Sicheng-Pan

rust/index/src/fulltext/types.rs

@@ -325,7 +325,7 @@ impl<'me> FullTextIndexReader<'me> {
             .then(|token| async move {
                 let positional_posting_list = self
                     .posting_lists_blockfile_reader
-                    .get_range(token.text.as_str()..=token.text.as_str(), ..)
+                    .get_prefix(token.text.as_str())
                     .await?
                     .collect::<Vec<_>>();
                 Ok::<_, FullTextIndexError>(positional_posting_list)
@@ -346,7 +346,7 @@ impl<'me> FullTextIndexReader<'me> {
                 .enumerate()
                 .map(|(i, posting_list)| {
                     if pointers[i] < posting_list.len() {
-                        Some(posting_list[pointers[i]].1)
+                        Some(posting_list[pointers[i]].0)
                     } else {
                         None
                     }
@@ -372,15 +372,15 @@ impl<'me> FullTextIndexReader<'me> {
 
                 // Seed with the positions of the first token.
                 let mut adjusted = posting_lists[0][pointers[0]]
-                    .2
+                    .1
                     .iter()
                     .copied()
                     .collect::<HashSet<_>>();
 
                 for i in 1..num_tokens {
                     let byte_delta_from_first_token =
                         tokens[i].offset_from as u32 - tokens[0].offset_from as u32;
-                    let positions = &posting_lists[i][pointers[i]].2;
+                    let positions = &posting_lists[i][pointers[i]].1;
 
                     let shifted = positions
                         .iter()
@@ -424,10 +424,10 @@ impl<'me> FullTextIndexReader<'me> {
     ) -> Result<Vec<(u32, Vec<u32>)>, FullTextIndexError> {
         let positional_posting_list = self
             .posting_lists_blockfile_reader
-            .get_range(token..=token, ..)
+            .get_prefix(token)
             .await?;
         let mut results = vec![];
-        for (_, doc_id, positions) in positional_posting_list {
+        for (doc_id, positions) in positional_posting_list {
             results.push((doc_id, positions.to_vec()));
         }
         Ok(results)