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/Cargo.toml

@@ -35,6 +35,7 @@ ui.workspace = true
 util.workspace = true
 workspace.workspace = true
 worktree.workspace = true
+open = { workspace = true }
 
 [dev-dependencies]
 collections = { workspace = true, features = ["test-support"] }