tooltip: Raise TooltipOverlay priority to float above dialog and popup layers (#2746)

Fixes an issue where tooltips triggered on elements inside a modal
`Dialog` or dropdown `Popup` are rendered underneath the dialog backdrop
and window.

`TooltipOverlay` previously rendered with a hardcoded
`with_priority(2)`. Because `Dialog` uses `with_priority(10 +
self.layer)` and `Popup` uses `with_priority(POPUP_PRIORITY)` (`100`),
any tooltip triggered inside a dialog or popup had a lower deferred
rendering priority than the dialog itself, causing the tooltip to be
occluded.

- Define and export `pub const TOOLTIP_PRIORITY: usize = 200;` in
`gpui_base::tooltip`.
- Define and export `pub const DIALOG_PRIORITY: usize = 10;` in
`gpui_base::dialog` to replace the magic number `10`.
- Update `TooltipOverlay::render` to use `TOOLTIP_PRIORITY`.
- Add unit test verifying `TOOLTIP_PRIORITY > POPUP_PRIORITY >
DIALOG_PRIORITY`.

`cargo test -p gpui-base`
`cargo test -p gpui-component`

Closes #[issue number]

## Description

Describe in English for the changes made in this pull request and the
problem it solves.
Please keep **1 PR to solve 1 problem**, and keep **Small improvements
should be small modifications** to make PR easier to review and to
merge.

## Screenshot

**Before**
<img width="216" height="105" alt="image"
src="https://github.com/user-attachments/assets/e05dab81-09d6-4e77-9238-3b0f0771262b"
/>

**After**
<img width="199" height="105" alt="image"
src="https://github.com/user-attachments/assets/3751fd4c-f627-4e78-b59a-37c41f909715"
/>

## Break Changes

None

## How to Test

Run the test suites across `gpui-base` and `gpui-component`, including
the newly added layer priority regression test:

```bash
# Run gpui-base tests (verifies tooltip_priority_exceeds_dialog_and_popup_layers)
cargo test -p gpui-base
````

## Checklist

- [ ] I have read the [CONTRIBUTING](../CONTRIBUTING.md) document and
followed the guidelines.
- [ ] Reviewed the changes in this PR and confirmed AI generated code
(If any) is accurate.
- [ ] Passed `cargo run` for story tests related to the changes.
- [ ] Tested macOS, Windows and Linux platforms performance (if the
change is platform-specific)