Return a not-found response for group SSH certs in FOSS

What does this MR do and why?

Follow-up from !253556 (comment 3820009080). The FOSS group_ssh_certificate_response returned nil, so authorized_cert_payload needed group_response&.success? and group_claimed_fingerprint? needed a nil guard.

This MR returns the same ServiceResponse.error the EE finder returns when nothing matches (Certificate Not Found, reason CERTIFICATE_NOT_FOUND). The callers then treat FOSS exactly like an EE instance with no matching group certificate, and both nil checks go away. The existing request specs for GET /internal/authorized_certs cover the not-found and user-not-found paths and are unchanged.

References

• Resolves #628758 (closed)
• Origin: !253556 (merged)

Differences

Before: FOSS returns nil from group_ssh_certificate_response and the callers special-case it.

After: FOSS returns a not-found error response; no behaviour change for any request.

How to set up and validate locally

1. Run bundle exec rspec spec/requests/api/internal/base_spec.rb -e authorized_certs.

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