Replace broken BBMs and add config validations

What does this MR do and why?

• Makes the changed BBMs a "no-op" due to a bug from using to_json to convert jsonb columns:
  • to_json was called on the data before saving, causing it to be saved as a literal string (which is valid JSON) instead of an object.
• Creates new BBMs that are 1:1 copies of the older code, outside of the .to_json change. As suggested from Slack: https://gitlab.slack.com/archives/C3NBYFJ6N/p1743613558709539
• Adds a validation step to the model concern for config being a hash

References

**Old original MRs: **

1. !184058 (merged)
2. !184039 (merged)
3. !184035 (merged)
4. !183044 (merged)
5. !183043 (merged)
6. !183042 (merged)

Screenshots or screen recordings

For checking the diffs between the current migration and the new migrations:

# 1. Amazon Instance
diff -u <(git show origin/master:ee/lib/ee/gitlab/background_migration/backfill_amazon_instance_audit_event_destinations.rb | grep -A 10 "def build_config") \
        <(grep -A 10 "def build_config" ee/lib/ee/gitlab/background_migration/backfill_amazon_instance_audit_event_destinations_fixed.rb)

# 2. Amazon Group
diff -u <(git show origin/master:ee/lib/ee/gitlab/background_migration/backfill_amazon_group_audit_event_destinations.rb | grep -A 10 "def build_config") \
        <(grep -A 10 "def build_config" ee/lib/ee/gitlab/background_migration/backfill_amazon_group_audit_event_destinations_fixed.rb)

# 3. External Instance
diff -u <(git show origin/master:ee/lib/ee/gitlab/background_migration/backfill_external_instance_audit_event_destinations.rb | grep -A 10 "def build_config") \
        <(grep -A 10 "def build_config" ee/lib/ee/gitlab/background_migration/backfill_external_instance_audit_event_destinations_fixed.rb)

# 4. External Group
diff -u <(git show origin/master:ee/lib/ee/gitlab/background_migration/backfill_external_group_audit_event_destinations.rb | grep -A 10 "def build_config") \
        <(grep -A 10 "def build_config" ee/lib/ee/gitlab/background_migration/backfill_external_group_audit_event_destinations_fixed.rb)

# 5. Google Instance
diff -u <(git show origin/master:ee/lib/ee/gitlab/background_migration/backfill_google_instance_audit_event_destinations.rb | grep -A 10 "def build_config") \
        <(grep -A 10 "def build_config" ee/lib/ee/gitlab/background_migration/backfill_google_instance_audit_event_destinations_fixed.rb)

# 6. Google Group
diff -u <(git show origin/master:ee/lib/ee/gitlab/background_migration/backfill_google_group_audit_event_destinations.rb | grep -A 10 "def build_config") \
        <(grep -A 10 "def build_config" ee/lib/ee/gitlab/background_migration/backfill_google_group_audit_event_destinations_fixed.rb)

How to set up and validate locally

Validating our config changes:

ActiveRecord::Base.logger = nil

normal_dest = AuditEvents::Group::ExternalStreamingDestination.new(
  group_id: Group.first.id,
  category: 'http',
  config: { url: FFaker::Internet.unique.http_url },
  secret_token: SecureRandom.hex(8)
)
normal_dest.valid?  
normal_dest.config.class  
normal_dest.save!


string_dest = AuditEvents::Group::ExternalStreamingDestination.new(
  group_id: Group.first.id,
  category: 'http',
  config: '{"url":"https://test.example.com"}',
  secret_token: SecureRandom.hex(8)
)
string_dest.valid? 
string_dest.config.class  
string_dest.save!


problematic = AuditEvents::Group::ExternalStreamingDestination.new(
  group_id: Group.first.id,
  category: 'http',
  name: 'Test String Config',
  secret_token: SecureRandom.hex(8)
)

problematic.send(:write_attribute, :config, '{"url":"https://example.com"}')

problematic.save(validate: false)
problematic_id = problematic.id


problematic = AuditEvents::Group::ExternalStreamingDestination.find(problematic_id)

puts "Config class: #{problematic.config.class}" 
puts "Config URL: #{problematic.config['url']}"  

problematic.name = "Updated Name"
puts "Valid after name change? #{problematic.valid?}"  
problematic.save!
puts "Save successful? #{problematic.persisted? && !problematic.changed?}"

raw_value = problematic.attributes_before_type_cast["config"]
puts "Raw stored config type: #{raw_value.class}"
puts "Raw stored config: #{raw_value}"

problematic.config = { url: 'https://updated-example.com' }
puts "Valid after config update? #{problematic.valid?}" 
problematic.save!

reloaded = AuditEvents::Group::ExternalStreamingDestination.find(problematic_id)
puts "Updated config URL: #{reloaded.config['url']}"