Fix chat input not scrolling to top with padding when Ctrl+Home is used

[Copilot]

When pressing Ctrl+Home in the chat input field, the cursor moves to the beginning of the buffer but the editor doesn't scroll properly to include the top padding area. This leaves a visible shadow artifact indicating there's more content above, even though the cursor is at the absolute beginning.

The issue occurs because the TopCommand uses VerticalRevealType.Simple which performs minimal scrolling and doesn't account for editor padding. Chat inputs have top padding configured ({ top: 8, bottom: 8 } or { top: 2, bottom: 2 } in compact mode), but this padding area remains hidden after Ctrl+Home.

This change modifies the TopCommand to detect when the cursor reaches the very beginning of the buffer (line 1, column 1) and uses VerticalRevealType.Top instead of the default reveal behavior. This ensures the editor scrolls to include the full padding area above the first line, eliminating the shadow artifact.

// Before: Always used default reveal behavior
viewModel.revealAllCursors(args.source, true);

// After: Use padding-aware reveal when at absolute beginning
const primaryCursor = newCursorStates[0];
if (primaryCursor && primaryCursor.modelState.position.lineNumber === 1 && primaryCursor.modelState.position.column === 1) {
    // Show full padding area
    const range = Range.fromPositions(primaryCursor.modelState.position);
    const viewRange = viewModel.coordinatesConverter.convertModelRangeToViewRange(range);
    viewModel.revealRange(args.source, true, viewRange, VerticalRevealType.Top, ScrollType.Smooth);
} else {
    // Default behavior for other positions
    viewModel.revealAllCursors(args.source, true);
}

The fix is targeted and safe - it only affects the specific case where Ctrl+Home moves the cursor to the absolute beginning, while maintaining existing behavior for all other cursor positions.

Fixes #260977.

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