Add Code Symbols tool (#27733)

Lets you get all the code symbols in the project (like the Code Symbols
panel) or in a particular file (like the Outline panel), optionally
paginated and filtering results by regex. The tool gives the files,
lines, and numbers of all of these, which means they can be used in
conjunction with the read file tool to read subsets of large files
without having to open the entire large file and poke around in it.

<img width="621" alt="Screenshot 2025-03-29 at 12 00 21 PM"
src="https://github.com/user-attachments/assets/d78259d7-2746-44c0-ac18-2e21f2505c0a"
/>

Release Notes:

- N/A

crates/assistant_tools/src/code_symbol_iter.rs

@@ -0,0 +1,88 @@
+use project::DocumentSymbol;
+use regex::Regex;
+
+#[derive(Debug, Clone)]
+pub struct Entry {
+    pub name: String,
+    pub kind: lsp::SymbolKind,
+    pub depth: u32,
+    pub start_line: usize,
+    pub end_line: usize,
+}
+
+/// An iterator that filters document symbols based on a regex pattern.
+/// This iterator recursively traverses the document symbol tree, incrementing depth for child symbols.
+#[derive(Debug, Clone)]
+pub struct CodeSymbolIterator<'a> {
+    symbols: &'a [DocumentSymbol],
+    regex: Option<Regex>,
+    // Stack of (symbol, depth) pairs to process
+    pending_symbols: Vec<(&'a DocumentSymbol, u32)>,
+    current_index: usize,
+    current_depth: u32,
+}
+
+impl<'a> CodeSymbolIterator<'a> {
+    pub fn new(symbols: &'a [DocumentSymbol], regex: Option<Regex>) -> Self {
+        Self {
+            symbols,
+            regex,
+            pending_symbols: Vec::new(),
+            current_index: 0,
+            current_depth: 0,
+        }
+    }
+}
+
+impl Iterator for CodeSymbolIterator<'_> {
+    type Item = Entry;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        if let Some((symbol, depth)) = self.pending_symbols.pop() {
+            for child in symbol.children.iter().rev() {
+                self.pending_symbols.push((child, depth + 1));
+            }
+
+            return Some(Entry {
+                name: symbol.name.clone(),
+                kind: symbol.kind,
+                depth,
+                start_line: symbol.range.start.0.row as usize,
+                end_line: symbol.range.end.0.row as usize,
+            });
+        }
+
+        while self.current_index < self.symbols.len() {
+            let regex = self.regex.as_ref();
+            let symbol = &self.symbols[self.current_index];
+            self.current_index += 1;
+
+            if regex.is_none_or(|regex| regex.is_match(&symbol.name)) {
+                // Push in reverse order to maintain traversal order
+                for child in symbol.children.iter().rev() {
+                    self.pending_symbols.push((child, self.current_depth + 1));
+                }
+
+                return Some(Entry {
+                    name: symbol.name.clone(),
+                    kind: symbol.kind,
+                    depth: self.current_depth,
+                    start_line: symbol.range.start.0.row as usize,
+                    end_line: symbol.range.end.0.row as usize,
+                });
+            } else {
+                // Even if parent doesn't match, push children to check them later
+                for child in symbol.children.iter().rev() {
+                    self.pending_symbols.push((child, self.current_depth + 1));
+                }
+
+                // Check if any pending children match our criteria
+                if let Some(result) = self.next() {
+                    return Some(result);
+                }
+            }
+        }
+
+        None
+    }
+}