test(config): service-label guard skips ServiceType constants not in typed-literal form #1722

Summary

TestEveryServiceTypeHasAFrontendLabel (internal/config/service_labels_frontend_test.go, added in #1710) collects the pinned library's service slugs by parsing common for constants written as Name ServiceType = "literal". A constant in any other form is skipped without failing the test:

• conversion form: X = ServiceType("x")
• an untyped value inside a typed const group

A slug added that way in a future library bump would get no frontend label, and the guard would still pass.

Expected

The guard also collects:

• a *ast.CallExpr whose Fun is the ServiceType ident and whose only argument is a string literal;
• any const spec in a group that inherits the ServiceType type.

Alternatively, it fails when it finds a ServiceType-typed constant whose value it cannot read, so a new form is reported instead of skipped.

Notes

• Every current ServiceType constant uses the explicit typed form, so nothing is missed today.
• Found by a mutation probe in the cold review of fix(frontend): show service display names instead of raw slugs #1710.