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/Cargo.toml

@@ -23,7 +23,6 @@ http_client.workspace = true
 itertools.workspace = true
 language.workspace = true
 language_model.workspace = true
-lsp.workspace = true
 project.workspace = true
 regex.workspace = true
 schemars.workspace = true