Add test report API route

What does this MR do?

This adds a route to get the JSON test report content, similar to how it's available via the UI.

It includes request specs and a minor update to the description of an existing related controller spec.

Blocked until !24188 (merged) is merged because of the changes to error handling and associated changes to the API.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

The JUnit report feature is currently off by default due to performance issues, which will be fixed in #37725 (closed)

Until then the new API route should also be behind the same feature flag as the full feature.

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team

Closes #202525 (closed)