Use tree-sitter when returning symbols to the model for a given file (#28352)

This also increases the threshold for when we return an outline during
`read_file`.

Release Notes:

- Fixed an issue that caused the agent to fail reading large files if
the LSP hadn't started yet.

crates/assistant_tools/src/assistant_tools.rs

@@ -1,6 +1,5 @@
 mod bash_tool;
 mod batch_tool;
-mod code_symbol_iter;
 mod code_symbols_tool;
 mod copy_path_tool;
 mod create_directory_tool;