Bar chart tooltips overlap with bars for high values

When using BarChart, tooltips for high-value bars overlap with the bars themselves, making them difficult to read.

**Steps to reproduce:**
1. Create a BarChart with some high value data points
2. Show tooltips on all bar rods using `showingTooltipIndicators: [0]` in `BarChartGroupData`
3. Notice tooltips appear on top of the bars, creating visual overlap

**Expected behavior:**
Tooltips should be positioned in a way that's clearly readable and doesn't overlap with the bar content

**Current behavior:**
Tooltips appear directly on top of tall bars, creating visual clutter and reducing readability

**Versions**
- Flutter version: 3.24.4
- fl_chart version: 0.69.0

**Screenshots:**

Start by reproducing the issue with BarChart and showingTooltipIndicators: [0] in BarChartGroupData using high-value data points. Inspect the BarChart tooltip positioning path and verify the result on Flutter 3.24.4. Done means tooltips remain clearly readable without overlapping tall bars.

Written by the indexing model from the issue text.