Build Gemini-native model contents for reasoning items when the routed model
uses the Gemini signature provider: thought summary on a thought part and the
replay signature on the adjacent visible part. Merge following assistant
visible text when it is not trailing prefill, and keep reasoning history when
stripping trailing model-authored prefill.
Extend Responses translator tests for signature compatibility and Antigravity
wrapper coverage; clarify stale replay signature expectations in executor tests.