assistant_tools: Add list-worktrees and read-file tools (#26147)

This PR adds two new tools to Assistant 2:

- `list-worktrees` - Lists the worktrees in a project
- `read-file` - Reads a file at the given path in the project

I don't see `list-worktrees` sticking around long-term, as when we have
tools for listing files those will include the worktree IDs along with
the path, but making this tool available allows the model to utilize
`read-file` when it otherwise wouldn't be able to.

Release Notes:

- N/A

crates/assistant_tools/src/read_file_tool.rs

@@ -0,0 +1,69 @@
+use std::path::Path;
+use std::sync::Arc;
+
+use anyhow::{anyhow, Result};
+use assistant_tool::Tool;
+use gpui::{App, Task, WeakEntity, Window};
+use project::{ProjectPath, WorktreeId};
+use schemars::JsonSchema;
+use serde::{Deserialize, Serialize};
+use workspace::Workspace;
+
+#[derive(Debug, Serialize, Deserialize, JsonSchema)]
+pub struct ReadFileToolInput {
+    /// The ID of the worktree in which the file resides.
+    pub worktree_id: usize,
+    /// The path to the file to read.
+    ///
+    /// This path is relative to the worktree root, it must not be an absolute path.
+    pub path: Arc<Path>,
+}
+
+pub struct ReadFileTool;
+
+impl Tool for ReadFileTool {
+    fn name(&self) -> String {
+        "read-file".into()
+    }
+
+    fn description(&self) -> String {
+        "Reads the content of a file specified by a worktree ID and path. Use this tool when you need to access the contents of a file in the project.".into()
+    }
+
+    fn input_schema(&self) -> serde_json::Value {
+        let schema = schemars::schema_for!(ReadFileToolInput);
+        serde_json::to_value(&schema).unwrap()
+    }
+
+    fn run(
+        self: Arc<Self>,
+        input: serde_json::Value,
+        workspace: WeakEntity<Workspace>,
+        _window: &mut Window,
+        cx: &mut App,
+    ) -> Task<Result<String>> {
+        let Some(workspace) = workspace.upgrade() else {
+            return Task::ready(Err(anyhow!("workspace dropped")));
+        };
+
+        let input = match serde_json::from_value::<ReadFileToolInput>(input) {
+            Ok(input) => input,
+            Err(err) => return Task::ready(Err(anyhow!(err))),
+        };
+
+        let project = workspace.read(cx).project().clone();
+        let project_path = ProjectPath {
+            worktree_id: WorktreeId::from_usize(input.worktree_id),
+            path: input.path,
+        };
+        cx.spawn(|cx| async move {
+            let buffer = cx
+                .update(|cx| {
+                    project.update(cx, |project, cx| project.open_buffer(project_path, cx))
+                })?
+                .await?;
+
+            cx.update(|cx| buffer.read(cx).text())
+        })
+    }
+}