Add Open Tool (#27499)

I've seen models try to run `open` in Bash. This is a cross-platform
version of that.

<img width="634" alt="Screenshot 2025-03-26 at 10 27 40 AM"
src="https://github.com/user-attachments/assets/b18cb50f-6e2f-4770-b15c-1040916a420a"
/>

Release Notes:

- N/A

crates/assistant_tools/src/open_tool.rs

@@ -0,0 +1,68 @@
+use anyhow::{anyhow, Context as _, Result};
+use assistant_tool::{ActionLog, Tool};
+use gpui::{App, AppContext, Entity, Task};
+use language_model::LanguageModelRequestMessage;
+use project::Project;
+use schemars::JsonSchema;
+use serde::{Deserialize, Serialize};
+use std::sync::Arc;
+use ui::IconName;
+use util::markdown::MarkdownString;
+
+#[derive(Debug, Serialize, Deserialize, JsonSchema)]
+pub struct OpenToolInput {
+    /// The path or URL to open with the default application.
+    path_or_url: String,
+}
+
+pub struct OpenTool;
+
+impl Tool for OpenTool {
+    fn name(&self) -> String {
+        "open".to_string()
+    }
+
+    fn needs_confirmation(&self) -> bool {
+        true
+    }
+
+    fn description(&self) -> String {
+        include_str!("./open_tool/description.md").to_string()
+    }
+
+    fn icon(&self) -> IconName {
+        IconName::ExternalLink
+    }
+
+    fn input_schema(&self) -> serde_json::Value {
+        let schema = schemars::schema_for!(OpenToolInput);
+        serde_json::to_value(&schema).unwrap()
+    }
+
+    fn ui_text(&self, input: &serde_json::Value) -> String {
+        match serde_json::from_value::<OpenToolInput>(input.clone()) {
+            Ok(input) => format!("Open `{}`", MarkdownString::escape(&input.path_or_url)),
+            Err(_) => "Open file or URL".to_string(),
+        }
+    }
+
+    fn run(
+        self: Arc<Self>,
+        input: serde_json::Value,
+        _messages: &[LanguageModelRequestMessage],
+        _project: Entity<Project>,
+        _action_log: Entity<ActionLog>,
+        cx: &mut App,
+    ) -> Task<Result<String>> {
+        let input: OpenToolInput = match serde_json::from_value(input) {
+            Ok(input) => input,
+            Err(err) => return Task::ready(Err(anyhow!(err))),
+        };
+
+        cx.background_spawn(async move {
+            open::that(&input.path_or_url).context("Failed to open URL or file path")?;
+
+            Ok(format!("Successfully opened {}", input.path_or_url))
+        })
+    }
+}