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/project/src/project.rs

@@ -10917,10 +10917,30 @@ impl<'a> fuzzy::PathMatchCandidateSet<'a> for PathMatchCandidateSet {
     }
 
     fn len(&self) -> usize {
-        if self.include_ignored {
-            self.snapshot.file_count()
-        } else {
-            self.snapshot.visible_file_count()
+        match self.candidates {
+            Candidates::Files => {
+                if self.include_ignored {
+                    self.snapshot.file_count()
+                } else {
+                    self.snapshot.visible_file_count()
+                }
+            }
+
+            Candidates::Directories => {
+                if self.include_ignored {
+                    self.snapshot.dir_count()
+                } else {
+                    self.snapshot.visible_dir_count()
+                }
+            }
+
+            Candidates::Entries => {
+                if self.include_ignored {
+                    self.snapshot.entry_count()
+                } else {
+                    self.snapshot.visible_entry_count()
+                }
+            }
         }
     }