assistant: Report all worktree entries in /file completions (#15617)

We were reporting file count as worktree entry count, which led to us
missing some of the entries in /file command completion.

/cc @bennetbo

The other components that used `PathMatchCandidateSet` are
`/diagnostics` and file finder. File finder is unaffected, as it used
`Candidates::Files` - thus previously reported count was correct for it;
`/diagnostics` were using `::Entries` as well, so it could miss entries
just like `/files`.

Release Notes:

- Fixed /file and /diagnostics slash commands omitting entries in it's
completions menu.

crates/worktree/src/worktree.rs

@@ -2118,6 +2118,24 @@ impl Snapshot {
         Ok(())
     }
 
+    pub fn entry_count(&self) -> usize {
+        self.entries_by_path.summary().count
+    }
+
+    pub fn visible_entry_count(&self) -> usize {
+        self.entries_by_path.summary().non_ignored_count
+    }
+
+    pub fn dir_count(&self) -> usize {
+        let summary = self.entries_by_path.summary();
+        summary.count - summary.file_count
+    }
+
+    pub fn visible_dir_count(&self) -> usize {
+        let summary = self.entries_by_path.summary();
+        summary.non_ignored_count - summary.non_ignored_file_count
+    }
+
     pub fn file_count(&self) -> usize {
         self.entries_by_path.summary().file_count
     }