References
[FR-28](https://linear.app/zed-industries/issue/FR-28/task-modal-does-not-show-runnable-rust-test).

The bug this PR aims to fix is

> I can click the play button beside a rust test function, but it does
not show up in the task modal.

I wasn't able to reproduce it, but I suspect this was caused by the task
system preferring LSP code actions by default. It checked that an LSP
was queried for a task instead of checking if the queried LSP actually
returned any tasks. So the fix was just adding the below if statement as
a check.

```rust
if !new_lsp_tasks.is_empty() {
    lsp_tasks
        .entry(source_kind)
        .or_insert_with(Vec::new)
        .append(&mut new_lsp_tasks);
}
```

I also added a regression test for this

Self-Review Checklist:

- [x] I have reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the UI/UX checklist
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable


Release Notes:

- Fixed task modal failing to show language tasks in some cases