Ensure no null settings context hash for extensions settings

Issue: Enforce not-null constraint on settings_context... (#503812 - closed)

What does this MR do and why?

• Adds not-null validation for extensions setting type at the application level only. This is because non-extensions setting types are expected to have null settings context hash.
• Backfills existing extensions settings with null settings context hash to default extensions marketplace settings.

References

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• Enforce not-null constraint on settings_context... (#503812 - closed)

MR acceptance checklist

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How to set up and validate locally

Setup:

Before pulling this branch, let's create vs_code_settings records in the Rails console:

Extension settings	Non-extension settings
VsCode::Settings::VsCodeSetting.create!( user_id: 1, setting_type: 'extensions', content:'{}', version: 6, settings_context_hash: nil, uuid: SecureRandom.uuid )	VsCode::Settings::VsCodeSetting.create!( user_id: 1, setting_type: 'globalState', content:'{}', version: 1, settings_context_hash: nil, uuid: SecureRandom.uuid )

Backfill:

• Pull this branch run migrations (bundle exec rake db:migrate)

• Confirm backfill successful: extensions setting no longer has nil settings context hash by running

  VsCode::Settings::VsCodeSetting.find_by(settings_context_hash: nil, setting_type: 'extensions') # nil

• Confirm non-extensions settings are not affected

  VsCode::Settings::VsCodeSetting.find_by(settings_context_hash: nil, setting_type: 'globalState') # returns record

Model Validation:

• Using the command outlined in the setup step, create another extensions settings record with nil settings context hash.
• Should result in validation error: ActiveRecord::RecordInvalid: Validation failed: Settings context hash cannot be blank for extensions setting type
• Create another non-extensions settings record with nil settings context hash and setting_type: 'keybindings'
• Should not result in validation error.