Go serializes ChatMessagePart.Text with omitempty, so empty
reasoning text (from reasoning_start with no delta) is omitted
from JSON. The frontend receives {type: "reasoning"} with text
as undefined, crashing on .trim() calls.
Mark Text as optional in the reasoning variant via the variants
struct tag. This generates ChatReasoningPart with text?: string
and the frontend falls back to "" via nullish coalescing.
Closes#23245