Reject incomplete JRuby cursor responses

[OskarEichler]

Summary

Return nil unless JRuby's cursor query receives a terminating response with exactly two coordinates.

Reproduction

Focused external cursor models feed EOF and incomplete single-coordinate responses. Current code returns partial/decremented arrays; the candidate rejects them.

Verification

• external EOF/incomplete/valid cursor models
• release/cumulative candidate: 36 tests / 143 assertions
• current upstream: 35 tests / 144 assertions
• syntax, package builds and Rails 8.1.3.1 loading

Compatibility

Valid row/column responses are unchanged. JRuby was modeled but is unavailable locally; current upstream CI is green on JRuby. Prepared with AI-assisted source review; no repository tests were changed.