PR 2: DI tests (Service, EspClient, runX, persistentPreRunE)

[AbsolutOD]

Summary

PR 2 of 2 implementing the DI and tests spec at docs/superpowers/specs/2026-05-08-di-and-tests-design.md.

This PR is purely additive — new tests against the seams introduced in PR 1 (#30). No production code changes.

• Commit 1 (test(ssm): cover Service methods via fake ssmAPI) — 13 TestService_* tests covering Save, GetOne, GetMany, Delete, Copy happy paths and AWS error mapping. Local fakeSSMAPI implements the unexported ssmAPI interface; pagination is exercised via a multi-page output sequence.
• Commit 2 (test(client): cover EspClient wrapper methods) — 11 TestEspClient_* tests covering all six wrapper methods. The non-trivial cases pin the composition order in Copy (re-fetch with decrypt) and Move (Get → Save → Delete with short-circuit on each failure).
• Commit 3 (test(cmd): cover runX functions via fake EspClient) — 21 TestRun* tests across all seven subcommands. cmd/testutil_test.go provides fakeBackend, newTestEspClient, newCmdWithFlags, testConfig shared across files; the fakeBackend is a deliberate copy of the one in internal/client/client_test.go (two callers, no shared package).
• Commit 4 (test(cmd): cover persistentPreRunE env-var validation) — 3 TestPersistentPreRunE_* tests covering missing-region, missing-profile, and unsupported-backend error paths. Custom unsetEnv helper because t.Setenv only sets.

Total: 48 new tests across 8 new/modified test files.

Coverage gaps explicitly accepted

Per the spec:

• ssm.New() — calls config.LoadDefaultConfig, environment-coupled.
• app.InitQuestions and cmd/init.go — interactive survey.Ask.
• cmd/get.go::display* — formatting only, no logic.
• cmd/root.go::configureLogging — sets slog default, no failure path.
• main.go — single line.

Test plan

• go build ./... clean
• go vet ./... clean
• go test ./... green; all 48 new tests pass
• No new third-party test dependencies (stdlib testing only)

Notes for review

• All fakes are local to their consuming package. fakeBackend is duplicated between internal/client/client_test.go and cmd/testutil_test.go rather than promoted to a shared clienttest package — appropriate for two callers, avoids exporting test-only API.
• One incidental finding (out of scope, flagged for future cleanup): internal/ssm/errors.go::checkSSMByPathError is dead code. The checkSSMError switch has no GetMany case, so it falls through to checkBaseSSMErrors. Caught because TestService_GetMany_ErrorMidIteration succeeds via InternalServerError being recognized by the base mapper, not the per-action one.