Add delete endpoint for Alert metric images

What does this MR do and why?

This introduces a Delete API for Alert Management Metric Images.

Previous MRs:

• Database & Models
• Read alerts endpoint
• Create / update endpoint

Delete images

Curl

curl --location --request DELETE 'http://gdk.test:3000/api/v4/projects/6/alert_management_alerts/12/metric_images/2' \
--header 'PRIVATE-TOKEN: glpat-DEpkrRZUMVgm6ixjRGCr' \
--form 'url="http://gitlab.com"'

Response

<204 response>

Screenshots or screen recordings

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How to set up and validate locally

How to set up and validate locally

First create an alert:

1. Navigate to Settings > Monitor > Alerts > Current integrations
2. Create & enable a new HTTP Endpoint alert integration if none exist
3. For an existing integration, click > Send test alert to trigger a test with a sample payload, like this one:
   • {"title":"Triggering a test alert to associate with an incident"}

Then, create a metric image:

file = Rack::Test::UploadedFile.new('spec/fixtures/rails_sample.jpg', 'image/jpg')

alert = AlertManagement::Alert.last

alert.metric_images.create(file: file, url: 'http://www.gitlab.com', url_text: 'Example metric')

Then call the API using the example above.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #340852 (closed)