#1384 test: fix narrowing byte fixtures in Windows conversation test

Description

The conversation split failure test initializes uint8_t vectors with runtime int expressions. MSVC rejects that list initialization as narrowing, which prevents the conversation test targets from building on Windows.

Cast the two generated fixture values to uint8_t explicitly. The loop only generates small representable values; the fault-injection coverage and engine code are unchanged.

Validation: built with MSVC 19.51 and ran all six conversation tests successfully on Windows, including the GPU snapshot test. The change is limited to the test fixture.