fix: only count bar dataset axes in bar width calculation for multiple x-axis

[godfengliang]

Fixes #12262

Problem

When a chart has both bar and line datasets on different x-axes, the bar width is incorrectly calculated. The _getAxis() method counts ALL dataset axes (including those used by non-bar datasets like line), causing stackCount * axisCount to be larger than necessary and bars to appear thinner than they should.

Root Cause

Commit 8ea47ca introduced _getAxis() to handle stacked bars with multiple x-axes. However, it iterates over ALL datasets regardless of type, collecting axes from line/scatter/etc. datasets that do not affect bar width.

Fix

Modified _getAxis() to skip datasets whose resolved type (meta.type) does not match the bar controller type (this._type). This ensures only axes containing bar datasets are counted, consistent with how _getStacks() already filters by type via getMatchingVisibleMetas(this._type).

Before: Axis count includes line dataset axes -> bars too thin
After: Axis count only includes bar dataset axes -> bars fill correct width

Testing

The existing test stacked-and-multiple-axis.js (all bar datasets) continues to work correctly since all datasets have type === 'bar'. Mixed charts (bar + line) now correctly exclude line dataset axes from the calculation.