Anthropic API returns 400 when multiple tool_result blocks share the same
tool_use_id, or when consecutive tool results are sent as separate user
messages. This fix:
1. Adds ToolCallID deduplication in sanitizeHistoryForProvider (context.go)
to drop duplicate tool results before sending to any provider.
2. Merges consecutive tool result messages into a single user message with
multiple tool_result content blocks in Anthropic's buildRequestBody,
for both "user" (with ToolCallID) and "tool" role messages.
3. Adds tests for both behaviors.
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Walk backwards over preceding tool messages to find the nearest assistant
with ToolCalls, instead of only checking the immediate predecessor. Add
unit tests for sanitizeHistoryForProvider covering key edge cases.
Liqiang Liu