Filter out invalid emails in Pipeline Emails integrations

What does this MR do and why?

Enhance email validation and logging for GitLab's integration system:

• Add email format validation for pipeline status email integration
• Filter out invalid emails to ensure only correctly formatted addresses receive notifications
• Add logging for integration type to aid monitoring and debugging
• Update documentation to reflect that integrations filter emails

References

#554881 (closed)

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Turn on the Pipeline status emails integration for a GitLab project with CI enabled. For example: https://gdk.test:3000/gitlab-org/gitlab-shell/-/settings/integrations
2. Add both a valid and an invalid email to the recipients list, then select all branches.
3. Run a pipeline.
4. Confirm that an email is received at the valid email address: https://gdk.test:3000/rails/letter_opener.
5. Inspect the Sidekiq logs to ensure the integration_class was logged for the Integrations::ExecuteWorker worker.

MR acceptance checklist

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