Resolve "Improve discoverability of location of unit test failures"

What does this MR do and why?

This MR introduces a new Rest endpoint that returns the test_summary for a single build.

Screenshots or screen recordings

How to set up and validate locally

1. Start a rails console and run the following commands to create some test data

• build = FactoryBot.create(:ci_build, :success, :report_results, project: Project.last)
• Ci::BuildReportResultService.new.execute(build)

2. Get the namespace, project_id and build_id from the build we just created
3. Ping the endpoint --> http://127.0.0.1:3000/<namespace>/<project_id>/-/jobs/<build_id>/test_report_summary.json

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 #404654 (closed)