[core] test: Add explicit messages when a template fails its validation

[cbentejac]

Description

This PR adds explicit messages that explain why the test that checks whether a template is valid has failed. Prior to it, only compatibility issues were clearly explained.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.91%. Comparing base (f7a48c9) to head (78033ab).
⚠️ Report is 55 commits behind head on develop.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #2937   +/-   ##
========================================
  Coverage    78.91%   78.91%           
========================================
  Files           59       59           
  Lines         8305     8305           
========================================
  Hits          6554     6554           
  Misses        1751     1751           

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[Copilot]

Pull Request Overview

This PR improves the error reporting in the checkTemplateVersions function by adding explicit error messages for three validation failure scenarios. Previously, only compatibility issues had clear error messages, while other validation failures returned False without explanation.

• Added informative print statements for invalid graph, non-template files, and unknown node types
• Updated the compatibility issue error message for consistency in formatting

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