ci: split chart-testing CI into per-chart matrix jobs [RHIDP-14729]

[rm3l]

Description of the change

Currently, both the PR (test.yaml) and nightly (nightly.yaml) workflows run ct install against all applicable charts in a single job. When a failure occurs:

• It is difficult to determine which chart caused the failure without scrolling through lengthy logs.
• A failure in one chart blocks visibility into the results of other charts.
• Re-running the workflow re-tests all charts, not just the one that failed.

Example: https://github.com/redhat-developer/rhdh-chart/actions/runs/27310055619/job/80677740243

This PR converts both workflows into dynamic per-chart matrix jobs so that failures are immediately attributable to a specific chart.

This will help with the new standalone chart that will be added soon.

Which issue(s) does this PR fix or relate to

• RHIDP-14729

How to test changes / Special notes to the reviewer

Example workflows:

• PR
• Nightly

Checklist

• N/A — this PR only changes CI workflows and config, no chart code was modified.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[rm3l]

Traceback (most recent call last):
  File "/opt/app-root/src/install-dynamic-plugins.py", line 1556, in <module>
    main()
  File "/opt/app-root/src/install-dynamic-plugins.py", line 1510, in main
    merge_plugin(plugin, all_plugins, dynamic_plugins_file, level=1)
  File "/opt/app-root/src/install-dynamic-plugins.py", line 151, in merge_plugin
    return OciPackageMerger(plugin, dynamic_plugins_file, all_plugins).merge_plugin(level)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/app-root/src/install-dynamic-plugins.py", line 611, in merge_plugin
    raise InstallException(
InstallException: Cannot use {{inherit}} for oci://registry.access.redhat.com/rhdh/red-hat-developer-hub-backstage-plugin-lightspeed: no existing plugin configuration found. Ensure a plugin from this image is defined in an included file with an explicit version.

======= Cleaning up temporary catalog index directory
======= Removed lock file: /dynamic-plugins-root/install-dynamic-plugins.lock

The test failures on the backstage chart seem similar to https://redhat.atlassian.net/browse/RHDHBUGS-3374, caused by wrong references to the Lightspeed plugins in the 1.10 catalog index image. So not related to this PR. Also seen on the nightly jobs: https://github.com/redhat-developer/rhdh-chart/actions/runs/27651979368/job/81777873172#step:4:1369
The same tests passed yesterday before the plugin catalog index image update.

[rm3l]

/override "Test (backstage)"
/override "Test Latest Release"

[openshift-ci]

@rm3l: Overrode contexts on behalf of rm3l: Test (backstage), Test Latest Release

Details

In response to this:

/override "Test (backstage)"
/override "Test Latest Release"

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