Fixes#774. When tool_result content contains multiple text blocks,
they were serialized as arrays instead of strings, causing DeepSeek
to reject the request with 400 error.
Changes:
- convertToolResultContent: collapse all-text arrays to joined string
- convertContentBlocks: defensive collapse for user/assistant messages
- Arrays with images are preserved (not collapsed)
Tests: 3 new tests added, 53 pass, 0 fail
Co-authored-by: nick.mesen <nickmesen@users.noreply.github.com>
761924daa7