Unable to save text format without enabling Markdown filter

I think this particular error is a result of config schema validation by Durpal core's ValidKeysConstraint, see change record https://www.drupal.org/node/3324853 →

I think the solution to this issue could be along the lines of the following:

1. Refine the Markdown filter plugin form and configuration to remove vertical_tabs from the saved form_state
I think when saving the filter without Markdown enabled, vertical_tabs is left in the form_state and it is being saved with the markdown filter settings; however, vertical_tabs is not a key defined with the filter_settings.markdown schema.

2. Further define the schema for the markdown filter settings

Here's how Drupal core's filter_html schema is defined here

filter_settings.filter_html:
  type: mapping
  label: 'Filter HTML'
  mapping:
    allowed_html:
      type: string
      label: 'Allowed HTML'
    filter_html_help:
      type: boolean
      label: 'HTML help'
    filter_html_nofollow:
      type: boolean
      label: 'HTML nofollow'

Compared to the markdown filter schema here

# Filter settings.
filter_settings.markdown:
  type: markdown.parser
  mapping:
    override:
      type: boolean
      label: 'Override'

This patch prevents that `vertical_tabs` is added to the form state when the Markdown-filter is not enabled, which only addresses the first issue that @jasonawant suggests, but it's enough to prevent the error.

I was still getting errors like

'error' is not a supported key.
'parser' is not a supported key.

Fixed by the patch attached.