workbench.action.chat.nextUserPrompt/previousUserPrompt reveal the target prompt at inconsistent viewport positions

Does this issue occur when all extensions are disabled?: Yes/No

- VS Code Version: VS Code Version: 1.128.0
- OS Version: Windows 11

Steps to Reproduce:

1. Open the Chat view with a long conversation (several requests/responses taller than the viewport).
2. Run `workbench.action.chat.nextUserPrompt` (`Ctrl+Alt+Down`) repeatedly.
3. Run `workbench.action.chat.previousUserPrompt` (`Ctrl+Alt+Up`) repeatedly.

## Current Behavior

The two commands land the target prompt at **different viewport edges depending on navigation direction**, because the reveal is minimal-scroll:

- `nextUserPrompt`: target is below the viewport -> revealed aligned to the **bottom**, with its response (the content you typically want to read next) still off-screen.
- `previousUserPrompt`: target is above the viewport -> revealed aligned to the **top**.

So the same "jump to a user prompt" gesture places the focused item in opposite screen positions, and the eye must re-locate it on every jump.

## Expected Behavior

Both commands should reveal the target prompt at a **consistent, predictable viewport position regardless of direction** - e.g., always aligned to the top (so the prompt plus its following response read naturally downward), or any fixed relative position. The specific position matters less than the consistency.

## Pointer

The asymmetry comes from calling `reveal` without a position argument:

https://github.com/microsoft/vscode/blob/479de2c703c90021e41097ba0c69db0470a0a74d/src/vs/workbench/contrib/chat/browser/actions/chatPromptNavigationActions.ts#L119

`ChatWidget.reveal(item, relativeTop)` already forwards `relativeTop` to `ListView.reveal`, which supports fixed proportional positioning (`0` = top, `1` = bottom). Passing any constant makes both directions land identically.