fix: Added meaningful error msg for consumergroups

[shivaygupta-dotcom]

Summary

When syncing consumers that reference consumer groups, if those groups aren’t present in the state, the error message isn’t very helpful. It doesn’t clearly indicate which entity is missing, as it does not provide clear details about the missing entity.
more details here : https://kongstrong.slack.com/archives/C04349E4KRC/p1772639980263509

Issues resolved

Fix for Kong/deck#1916

Documentation

• Link to the website documentation PR

Testing

• Unit tests
• E2E tests
• Manual testing on Universal
• Manual testing on Kubernetes

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 28.89%. Comparing base (afd6c37) to head (cf43850).

Files with missing lines	Patch %	Lines
pkg/file/builder.go	0.00%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #442      +/-   ##
==========================================
- Coverage   28.90%   28.89%   -0.02%     
==========================================
  Files         121      121              
  Lines       16524    16530       +6     
==========================================
  Hits         4777     4777              
- Misses      11129    11135       +6     
  Partials      618      618              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.