Root cause: StreamConverter.Process() only incremented contentIndex when
closing a thinking block if text content was present. When a model emitted
thinking followed directly by a tool_use block (no text in between),
thinkingDone was never set and contentIndex was not incremented, causing the
tool_use content_block_start to reuse index 0. Clients expecting sequential
indices would then fail to find the tool content block.
Fix: In the tool call loop, close any open thinking block (thinkingStarted &&
!thinkingDone) and increment contentIndex before opening the tool_use block,
mirroring the existing logic that closes an open text block.
Fixes#14816
f3f31a8192