Add regression test for 'ds-records create' exit code

[ggalmazor]

Description

Follow-up from #20. That bug caused dnsimple domains ds-records create to exit 1 and print Error: Failed to create DS record. Please contact support while the record was actually being persisted server-side. The underlying cause was fixed in the API, but the CLI itself has no test that would catch a future recurrence of the same exit-code-vs-side-effect divergence on this command.

The CLI is a thin wrapper (internal/cli/domains_ds_records.go, newDsRecordsCreateCmd) that simply propagates errors from c.Domains.CreateDelegationSignerRecord(...). A small integration-style test that exercises the happy path (stubbed or sandbox-backed) and asserts exit code 0 with no Error: line on stderr would have caught the original regression immediately and would protect the next DS-records callers (automation, CI scripts) from silent exit-code breakage.

Expected Behavior or Outcome

• A regression test exists for the ds-records create command that fails if the command exits non-zero on a successful create.
• The test runs in CI on every PR.
• The test is written in a style that is straightforward to replicate for the sibling DS-records subcommands (list, delete) if they gain similar coverage later.

Acceptance Criteria

• A test exercises dnsimple domains ds-records create against a stubbed/fake DNSimple API that responds with a 2xx and a valid DS record body, and asserts the command exits with code 0.
• The test asserts that no line starting with Error: is emitted on stderr for the happy path.
• The test runs as part of the default go test ./... (or equivalent CI invocation) and does not require any out-of-band setup beyond what other CLI tests already require.
• The test is documented briefly in its file header so a reader understands what regression it guards.

Resources/References

• Original bug: domains ds-records create returns error but creates the record #20
• Affected CLI file: internal/cli/domains_ds_records.go (newDsRecordsCreateCmd)

Notes

Exit-code correctness is the load-bearing property here. The assertion does not need to validate output formatting beyond "no error line"; stricter assertions on the exact stdout shape can live in a separate, more focused test.