Problem

Markdown and JSON display objects had no __repr__, so they fell
back to DisplayObject.__repr__ which returns the unhelpful
<IPython.core.display.Markdown object> string. This caused nbconvert
output and terminal reprs to show garbage instead of actual content.

Changes

• Add Markdown.__repr__ returning self.data
• Add JSON.__repr__ returning json.dumps(self.data, indent=2)
  (as requested by @Carreau in Add a 'text/plain' repr to Markdown and JSON #14150)
• Update test_geojson to assert the correct repr output

Notes

• Builds on the approach in Add a 'text/plain' repr to Markdown and JSON #14150 and directly addresses the
  maintainer feedback requesting json.dumps with indentation
• test_display_formatter_active_types_config failure is a
  pre-existing Windows environment issue unrelated to this PR

Closes #12895