Gemini can attach thoughtSignature to a part whose visible text is
empty and requires it echoed back; convertMessages dropped such blocks
together with the signature, breaking the reasoning chain. With the
chain broken, gemini flash models intermittently end a mid-task turn
with a thought-only STOP and no tool call. Empty blocks are now dropped
only when they are unsigned, mirroring the anthropic converter.
Co-authored-by: Jingtao Xu <jingtaozf@gmail.com>
