fix: disable autoSkip when stepSize is explicitly set

[Sonali-Sharma-tech]

Summary

When ticks.stepSize is set, autoSkip (enabled by default) can still hide ticks — contradicting the user's explicit step size configuration. Users currently have to manually set autoSkip: false alongside stepSize, which is unintuitive.

This fix adds a !tickOpts.stepSize check to the autoSkip condition in core.scale.js, so that autoSkip is automatically disabled when stepSize is explicitly set.

Reproduction

options: {
  scales: {
    y: {
      ticks: {
        stepSize: 1  // expects every tick, but autoSkip hides some
      }
    }
  }
}

Before fix: Some ticks are hidden by autoSkip despite stepSize: 1
After fix: All ticks are shown at the specified interval

Changes

• src/core/core.scale.js — added !tickOpts.stepSize guard to autoSkip condition
• test/specs/scale.linear.tests.js — added test verifying all ticks are visible when stepSize is set

Backward compatibility

• No breaking changes — explicitly setting autoSkip: false still works as before
• Only affects behavior when stepSize is set (autoSkip is skipped automatically)

Fixes #4048