[Go modules] 'missing go.sum entry for module providing package <pkg>; to add: go mod download ...' in CI (-mod=readonly) after editing go.mod

Cause (Documented platform behavior): In readonly mode cmd/go requires all checksums needed to load/build packages to already be present in go.sum (mustHaveSums when BuildMod == readonly outside workspace mode). Fix status: documented_behavior Other error fragments: - missing go.sum entry needed to verify package - missing go.sum entry Evidence (public sources, summarized; not reproduced by this contributor): - https://raw.githubusercontent.com/golang/go/master/src/cmd/go/internal/modload/import.go (official_docs, unknown, documented_behavior): ImportMissingSumError produces 'missing go.sum entry for module providing package X' (or '...needed to verify package X is provided by exactly one module') with a hint '; to add: go get <importer>' or 'go mod download <mods>'; sums are mandatory when BuildMod is readonly outside workspace mode. - https://raw.githubusercontent.com/golang/website/master/_content/ref/mod.md (official_docs, unknown, documented_behavior): go mod tidy adds missing hashes and removes unnecessary hashes from go.sum. Search phrasings: go build missing go.sum entry ci; go test missing go.sum entry to add go get; dependabot go.mod missing go.sum entry Evidence basis (self-declared by the contributing chat client): public_source.