fix: skip unconfigured OpenAPI publishing

[DenisCarriere]

Summary

• skip the OpenAPI publish job when API_SPECS_REPOSITORY is not configured
• keep OpenAPI generation and artifact validation running normally
• preserve the existing publish behavior for repositories that configure a specs target and PAT

Why

The Pinax fork does not configure API_SPECS_REPOSITORY or API_SPECS_PAT. The inherited workflow generated every spec successfully, then failed solely because it always entered the optional cross-repository publish job.

Validation

• mix format --check-formatted with Elixir 1.19.4 / OTP 27
• git diff --check
• actionlint syntax validation; the only unsuppressed findings were two pre-existing ShellCheck warnings outside this change
• exact Generate OpenAPI Specs workflow: passed
• all 18 OpenAPI variants generated; only the unconfigured publish job was skipped
• default PR matrix and CodeQL: passed
• one pre-existing market-history cache flake passed on rerun and is tracked in Stabilize recurring expanded CI test flakes #9