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>
