[quality] Add coverage for workloads cluster group CRUD/sync handlers

[Copilot]

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📌 Fixes

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📝 Summary of Changes

pkg/api/handlers/workloads/cluster_groups.go had effectively no direct test coverage across cluster-group CRUD/sync behavior, including admin gating, validation, and persistence interactions. This PR replaces the placeholder test with focused handler-level coverage for the high-risk paths called out in the issue.

• HTTP behavior coverage

  • Validates built-in all-healthy-clusters group is prepended in list responses.
  • Adds create-path validation coverage (empty name, reserved name, missing clusters for static groups, dynamic-group allowance).

• Authorization and mutation safety

  • Adds table-driven 403 coverage for non-admin access across create/update/delete/sync endpoints.
  • Ensures reserved-name filtering in sync payloads and oversized-body rejection.

• Persistence contract checks

  • Verifies SaveClusterGroup on create/update and DeleteClusterGroup on delete with explicit mock expectations.
  • Adds deterministic cluster-group state reset helpers to avoid test cross-talk on global in-memory state.

Example assertion pattern used for persistence payload correctness:

mockStore.On("SaveClusterGroup", "persist-me", mock.MatchedBy(func(data []byte) bool {
	var group ClusterGroup
	if err := json.Unmarshal(data, &group); err != nil {
		return false
	}
	return group.Name == "persist-me" && group.Kind == "static"
})).Return(nil).Once()

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Changes Made

• Updated pkg/api/handlers/workloads/cluster_groups_test.go to replace placeholder coverage with real handler tests
• Refactored test setup with focused helpers (resetClusterGroups, newJSONRequest) for isolation and readability
• Fixed coverage gaps for admin enforcement, validation branches, reserved-name handling, and request-size guard
• Added tests for list/create/update/delete/sync and persistence side effects

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Checklist

Please ensure the following before submitting your PR:

• I used a coding agent (Claude Code, Copilot, Gemini, or Codex) to generate/review this code
• I have reviewed the project's contribution guidelines
• New cards target console-marketplace, not this repo
• isDemoData is wired correctly (cards show Demo badge when using demo data)
• I have written unit tests for the changes (if applicable)
• I have tested the changes locally and ensured they work as expected
• All commits are signed with DCO (git commit -s)

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Screenshots or Logs (if applicable)

N/A (Go handler test updates only)

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👀 Reviewer Notes

• Scope is intentionally surgical: one test file replaced/expanded; no production handler logic changed.
• Existing unrelated package test failures remain outside this PR’s change scope.

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