Add Kustomize fixture scenarios and e2e coverage

[whitemerch]

Summary

• add Kustomize fixture trees for canonical, regression, and scan-scenario coverage
• add scenario golden expectations and dedicated test coverage (TestScanScenarios, TestKustomizeEndToEnd)
• align payload fixture formatting for stable e2e assertions

Full design context and architecture details: #113

Test plan

• go test ./test -run 'TestKustomizeEndToEnd|TestScanScenarios' -count=1
• go test ./e2e -run 'Test_E2E_CLI/should_create_a_payload_file' -count=1
• $(go env GOPATH)/bin/golangci-lint run ./test/... ./e2e/...

Notes

• stacked on top of PR Wire Kustomize activation through scan and CLI #117
• this PR is fixtures/tests only

[datadog-datadog-prod-us1]

🎯 Code Coverage (details)
• Patch Coverage: 100.00%
• Overall Coverage: 47.35% (+0.05%)

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